linux/kernel/posix-cpu-timers.c

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/*
* Implement CPU time clocks for the POSIX clock interface.
*/
#include <linux/sched.h>
#include <linux/posix-timers.h>
#include <linux/errno.h>
#include <linux/math64.h>
#include <asm/uaccess.h>
static int check_clock(const clockid_t which_clock)
{
int error = 0;
struct task_struct *p;
const pid_t pid = CPUCLOCK_PID(which_clock);
if (CPUCLOCK_WHICH(which_clock) >= CPUCLOCK_MAX)
return -EINVAL;
if (pid == 0)
return 0;
read_lock(&tasklist_lock);
p = find_task_by_vpid(pid);
if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
same_thread_group(p, current) : thread_group_leader(p))) {
error = -EINVAL;
}
read_unlock(&tasklist_lock);
return error;
}
static inline union cpu_time_count
timespec_to_sample(const clockid_t which_clock, const struct timespec *tp)
{
union cpu_time_count ret;
ret.sched = 0; /* high half always zero when .cpu used */
if (CPUCLOCK_WHICH(which_clock) == CPUCLOCK_SCHED) {
ret.sched = (unsigned long long)tp->tv_sec * NSEC_PER_SEC + tp->tv_nsec;
} else {
ret.cpu = timespec_to_cputime(tp);
}
return ret;
}
static void sample_to_timespec(const clockid_t which_clock,
union cpu_time_count cpu,
struct timespec *tp)
{
if (CPUCLOCK_WHICH(which_clock) == CPUCLOCK_SCHED)
*tp = ns_to_timespec(cpu.sched);
else
cputime_to_timespec(cpu.cpu, tp);
}
static inline int cpu_time_before(const clockid_t which_clock,
union cpu_time_count now,
union cpu_time_count then)
{
if (CPUCLOCK_WHICH(which_clock) == CPUCLOCK_SCHED) {
return now.sched < then.sched;
} else {
return cputime_lt(now.cpu, then.cpu);
}
}
static inline void cpu_time_add(const clockid_t which_clock,
union cpu_time_count *acc,
union cpu_time_count val)
{
if (CPUCLOCK_WHICH(which_clock) == CPUCLOCK_SCHED) {
acc->sched += val.sched;
} else {
acc->cpu = cputime_add(acc->cpu, val.cpu);
}
}
static inline union cpu_time_count cpu_time_sub(const clockid_t which_clock,
union cpu_time_count a,
union cpu_time_count b)
{
if (CPUCLOCK_WHICH(which_clock) == CPUCLOCK_SCHED) {
a.sched -= b.sched;
} else {
a.cpu = cputime_sub(a.cpu, b.cpu);
}
return a;
}
[PATCH] posix-cpu-timers: prevent signal delivery starvation The integer divisions in the timer accounting code can round the result down to 0. Adding 0 is without effect and the signal delivery stops. Clamp the division result to minimum 1 to avoid this. Problem was reported by Seongbae Park <spark@google.com>, who provided also an inital patch. Roland sayeth: I have had some more time to think about the problem, and to reproduce it using Toyo's test case. For the record, if my understanding of the problem is correct, this happens only in one very particular case. First, the expiry time has to be so soon that in cputime_t units (usually 1s/HZ ticks) it's < nthreads so the division yields zero. Second, it only affects each thread that is so new that its CPU time accumulation is zero so now+0 is still zero and ->it_*_expires winds up staying zero. For the VIRT and PROF clocks when cputime_t is tick granularity (or the SCHED clock on configurations where sched_clock's value only advances on clock ticks), this is not hard to arrange with new threads starting up and blocking before they accumulate a whole tick of CPU time. That's what happens in Toyo's test case. Note that in general it is fine for that division to round down to zero, and set each thread's expiry time to its "now" time. The problem only arises with thread's whose "now" value is still zero, so that now+0 winds up 0 and is interpreted as "not set" instead of ">= now". So it would be a sufficient and more precise fix to just use max(ticks, 1) inside the loop when setting each it_*_expires value. But, it does no harm to round the division up to one and always advance every thread's expiry time. If the thread didn't already fire timers for the expiry time of "now", there is no expectation that it will do so before the next tick anyway. So I followed Thomas's patch in lifting the max out of the loops. This patch also covers the reload cases, which are harder to write a test for (and I didn't try). I've tested it with Toyo's case and it fixes that. [toyoa@mvista.com: fix: min_t -> max_t] Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@elte.hu> Signed-off-by: Roland McGrath <roland@redhat.com> Cc: Daniel Walker <dwalker@mvista.com> Cc: Toyo Abe <toyoa@mvista.com> Cc: john stultz <johnstul@us.ibm.com> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: Seongbae Park <spark@google.com> Cc: Peter Mattis <pmattis@google.com> Cc: Rohit Seth <rohitseth@google.com> Cc: Martin Bligh <mbligh@google.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-17 09:09:39 +02:00
/*
* Divide and limit the result to res >= 1
*
* This is necessary to prevent signal delivery starvation, when the result of
* the division would be rounded down to 0.
*/
static inline cputime_t cputime_div_non_zero(cputime_t time, unsigned long div)
{
cputime_t res = cputime_div(time, div);
return max_t(cputime_t, res, 1);
}
/*
* Update expiry time from increment, and increase overrun count,
* given the current clock sample.
*/
static void bump_cpu_timer(struct k_itimer *timer,
union cpu_time_count now)
{
int i;
if (timer->it.cpu.incr.sched == 0)
return;
if (CPUCLOCK_WHICH(timer->it_clock) == CPUCLOCK_SCHED) {
unsigned long long delta, incr;
if (now.sched < timer->it.cpu.expires.sched)
return;
incr = timer->it.cpu.incr.sched;
delta = now.sched + incr - timer->it.cpu.expires.sched;
/* Don't use (incr*2 < delta), incr*2 might overflow. */
for (i = 0; incr < delta - incr; i++)
incr = incr << 1;
for (; i >= 0; incr >>= 1, i--) {
if (delta < incr)
continue;
timer->it.cpu.expires.sched += incr;
timer->it_overrun += 1 << i;
delta -= incr;
}
} else {
cputime_t delta, incr;
if (cputime_lt(now.cpu, timer->it.cpu.expires.cpu))
return;
incr = timer->it.cpu.incr.cpu;
delta = cputime_sub(cputime_add(now.cpu, incr),
timer->it.cpu.expires.cpu);
/* Don't use (incr*2 < delta), incr*2 might overflow. */
for (i = 0; cputime_lt(incr, cputime_sub(delta, incr)); i++)
incr = cputime_add(incr, incr);
for (; i >= 0; incr = cputime_halve(incr), i--) {
if (cputime_lt(delta, incr))
continue;
timer->it.cpu.expires.cpu =
cputime_add(timer->it.cpu.expires.cpu, incr);
timer->it_overrun += 1 << i;
delta = cputime_sub(delta, incr);
}
}
}
static inline cputime_t prof_ticks(struct task_struct *p)
{
return cputime_add(p->utime, p->stime);
}
static inline cputime_t virt_ticks(struct task_struct *p)
{
return p->utime;
}
static inline unsigned long long sched_ns(struct task_struct *p)
{
return task_sched_runtime(p);
}
int posix_cpu_clock_getres(const clockid_t which_clock, struct timespec *tp)
{
int error = check_clock(which_clock);
if (!error) {
tp->tv_sec = 0;
tp->tv_nsec = ((NSEC_PER_SEC + HZ - 1) / HZ);
if (CPUCLOCK_WHICH(which_clock) == CPUCLOCK_SCHED) {
/*
* If sched_clock is using a cycle counter, we
* don't have any idea of its true resolution
* exported, but it is much more than 1s/HZ.
*/
tp->tv_nsec = 1;
}
}
return error;
}
int posix_cpu_clock_set(const clockid_t which_clock, const struct timespec *tp)
{
/*
* You can never reset a CPU clock, but we check for other errors
* in the call before failing with EPERM.
*/
int error = check_clock(which_clock);
if (error == 0) {
error = -EPERM;
}
return error;
}
/*
* Sample a per-thread clock for the given task.
*/
static int cpu_clock_sample(const clockid_t which_clock, struct task_struct *p,
union cpu_time_count *cpu)
{
switch (CPUCLOCK_WHICH(which_clock)) {
default:
return -EINVAL;
case CPUCLOCK_PROF:
cpu->cpu = prof_ticks(p);
break;
case CPUCLOCK_VIRT:
cpu->cpu = virt_ticks(p);
break;
case CPUCLOCK_SCHED:
cpu->sched = sched_ns(p);
break;
}
return 0;
}
/*
* Sample a process (thread group) clock for the given group_leader task.
* Must be called with tasklist_lock held for reading.
* Must be called with tasklist_lock held for reading, and p->sighand->siglock.
*/
static int cpu_clock_sample_group_locked(unsigned int clock_idx,
struct task_struct *p,
union cpu_time_count *cpu)
{
struct task_struct *t = p;
switch (clock_idx) {
default:
return -EINVAL;
case CPUCLOCK_PROF:
cpu->cpu = cputime_add(p->signal->utime, p->signal->stime);
do {
cpu->cpu = cputime_add(cpu->cpu, prof_ticks(t));
t = next_thread(t);
} while (t != p);
break;
case CPUCLOCK_VIRT:
cpu->cpu = p->signal->utime;
do {
cpu->cpu = cputime_add(cpu->cpu, virt_ticks(t));
t = next_thread(t);
} while (t != p);
break;
case CPUCLOCK_SCHED:
cpu->sched = p->signal->sum_sched_runtime;
/* Add in each other live thread. */
while ((t = next_thread(t)) != p) {
cpu->sched += t->se.sum_exec_runtime;
}
[PATCH] Fix posix-cpu-timers sched_time accumulation I've spent the past 3 days digging into a glibc testsuite failure in current CVS, specifically libc/rt/tst-cputimer1.c The thr1 and thr2 timers fire too early in the second pass of this test. The second pass is noteworthy because it makes use of intervals, whereas the first pass does not. All throughout the posix-cpu-timers.c code, the calculation of the process sched_time sum is implemented roughly as: unsigned long long sum; sum = tsk->signal->sched_time; t = tsk; do { sum += t->sched_time; t = next_thread(t); } while (t != tsk); In fact this is the exact scheme used by check_process_timers(). In the case of check_process_timers(), current->sched_time has just been updated (via scheduler_tick(), which is invoked by update_process_times(), which subsequently invokes run_posix_cpu_timers()) So there is no special processing necessary wrt. that. In other contexts, we have to allot for the fact that tsk->sched_time might be a bit out of date if we are current. And the posix-cpu-timers.c code uses current_sched_time() to deal with that. Unfortunately it does so in an erroneous and inconsistent manner in one spot which is what results in the early timer firing. In cpu_clock_sample_group_locked(), it does this: cpu->sched = p->signal->sched_time; /* Add in each other live thread. */ while ((t = next_thread(t)) != p) { cpu->sched += t->sched_time; } if (p->tgid == current->tgid) { /* * We're sampling ourselves, so include the * cycles not yet banked. We still omit * other threads running on other CPUs, * so the total can always be behind as * much as max(nthreads-1,ncpus) * (NSEC_PER_SEC/HZ). */ cpu->sched += current_sched_time(current); } else { cpu->sched += p->sched_time; } The problem is the "p->tgid == current->tgid" test. If "p" is not current, and the tgids are the same, we will add the process t->sched_time twice into cpu->sched and omit "p"'s sched_time which is very very very wrong. posix-cpu-timers.c has a helper function, sched_ns(p) which takes care of this, so my fix is to use that here instead of this special tgid test. The fact that current can be one of the sub-threads of "p" points out that we could make things a little bit more accurate, perhaps by using sched_ns() on every thread we process in these loops. It also points out that we don't use the most accurate value for threads in the group actively running other cpus (and this is mentioned in the comment). But that is a future enhancement, and this fix here definitely makes sense. Signed-off-by: David S. Miller <davem@davemloft.net> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-01-07 00:36:48 +01:00
cpu->sched += sched_ns(p);
break;
}
return 0;
}
/*
* Sample a process (thread group) clock for the given group_leader task.
* Must be called with tasklist_lock held for reading.
*/
static int cpu_clock_sample_group(const clockid_t which_clock,
struct task_struct *p,
union cpu_time_count *cpu)
{
int ret;
unsigned long flags;
spin_lock_irqsave(&p->sighand->siglock, flags);
ret = cpu_clock_sample_group_locked(CPUCLOCK_WHICH(which_clock), p,
cpu);
spin_unlock_irqrestore(&p->sighand->siglock, flags);
return ret;
}
int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
{
const pid_t pid = CPUCLOCK_PID(which_clock);
int error = -EINVAL;
union cpu_time_count rtn;
if (pid == 0) {
/*
* Special case constant value for our own clocks.
* We don't have to do any lookup to find ourselves.
*/
if (CPUCLOCK_PERTHREAD(which_clock)) {
/*
* Sampling just ourselves we can do with no locking.
*/
error = cpu_clock_sample(which_clock,
current, &rtn);
} else {
read_lock(&tasklist_lock);
error = cpu_clock_sample_group(which_clock,
current, &rtn);
read_unlock(&tasklist_lock);
}
} else {
/*
* Find the given PID, and validate that the caller
* should be able to see it.
*/
struct task_struct *p;
rcu_read_lock();
p = find_task_by_vpid(pid);
if (p) {
if (CPUCLOCK_PERTHREAD(which_clock)) {
if (same_thread_group(p, current)) {
error = cpu_clock_sample(which_clock,
p, &rtn);
}
} else {
read_lock(&tasklist_lock);
if (thread_group_leader(p) && p->signal) {
error =
cpu_clock_sample_group(which_clock,
p, &rtn);
}
read_unlock(&tasklist_lock);
}
}
rcu_read_unlock();
}
if (error)
return error;
sample_to_timespec(which_clock, rtn, tp);
return 0;
}
/*
* Validate the clockid_t for a new CPU-clock timer, and initialize the timer.
* This is called from sys_timer_create with the new timer already locked.
*/
int posix_cpu_timer_create(struct k_itimer *new_timer)
{
int ret = 0;
const pid_t pid = CPUCLOCK_PID(new_timer->it_clock);
struct task_struct *p;
if (CPUCLOCK_WHICH(new_timer->it_clock) >= CPUCLOCK_MAX)
return -EINVAL;
INIT_LIST_HEAD(&new_timer->it.cpu.entry);
new_timer->it.cpu.incr.sched = 0;
new_timer->it.cpu.expires.sched = 0;
read_lock(&tasklist_lock);
if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
if (pid == 0) {
p = current;
} else {
p = find_task_by_vpid(pid);
if (p && !same_thread_group(p, current))
p = NULL;
}
} else {
if (pid == 0) {
p = current->group_leader;
} else {
p = find_task_by_vpid(pid);
if (p && !thread_group_leader(p))
p = NULL;
}
}
new_timer->it.cpu.task = p;
if (p) {
get_task_struct(p);
} else {
ret = -EINVAL;
}
read_unlock(&tasklist_lock);
return ret;
}
/*
* Clean up a CPU-clock timer that is about to be destroyed.
* This is called from timer deletion with the timer already locked.
* If we return TIMER_RETRY, it's necessary to release the timer's lock
* and try again. (This happens when the timer is in the middle of firing.)
*/
int posix_cpu_timer_del(struct k_itimer *timer)
{
struct task_struct *p = timer->it.cpu.task;
int ret = 0;
if (likely(p != NULL)) {
read_lock(&tasklist_lock);
if (unlikely(p->signal == NULL)) {
/*
* We raced with the reaping of the task.
* The deletion should have cleared us off the list.
*/
BUG_ON(!list_empty(&timer->it.cpu.entry));
} else {
spin_lock(&p->sighand->siglock);
if (timer->it.cpu.firing)
ret = TIMER_RETRY;
else
list_del(&timer->it.cpu.entry);
spin_unlock(&p->sighand->siglock);
}
read_unlock(&tasklist_lock);
if (!ret)
put_task_struct(p);
}
return ret;
}
/*
* Clean out CPU timers still ticking when a thread exited. The task
* pointer is cleared, and the expiry time is replaced with the residual
* time for later timer_gettime calls to return.
* This must be called with the siglock held.
*/
static void cleanup_timers(struct list_head *head,
cputime_t utime, cputime_t stime,
unsigned long long sum_exec_runtime)
{
struct cpu_timer_list *timer, *next;
cputime_t ptime = cputime_add(utime, stime);
list_for_each_entry_safe(timer, next, head, entry) {
list_del_init(&timer->entry);
if (cputime_lt(timer->expires.cpu, ptime)) {
timer->expires.cpu = cputime_zero;
} else {
timer->expires.cpu = cputime_sub(timer->expires.cpu,
ptime);
}
}
++head;
list_for_each_entry_safe(timer, next, head, entry) {
list_del_init(&timer->entry);
if (cputime_lt(timer->expires.cpu, utime)) {
timer->expires.cpu = cputime_zero;
} else {
timer->expires.cpu = cputime_sub(timer->expires.cpu,
utime);
}
}
++head;
list_for_each_entry_safe(timer, next, head, entry) {
list_del_init(&timer->entry);
if (timer->expires.sched < sum_exec_runtime) {
timer->expires.sched = 0;
} else {
timer->expires.sched -= sum_exec_runtime;
}
}
}
/*
* These are both called with the siglock held, when the current thread
* is being reaped. When the final (leader) thread in the group is reaped,
* posix_cpu_timers_exit_group will be called after posix_cpu_timers_exit.
*/
void posix_cpu_timers_exit(struct task_struct *tsk)
{
cleanup_timers(tsk->cpu_timers,
tsk->utime, tsk->stime, tsk->se.sum_exec_runtime);
}
void posix_cpu_timers_exit_group(struct task_struct *tsk)
{
cleanup_timers(tsk->signal->cpu_timers,
cputime_add(tsk->utime, tsk->signal->utime),
cputime_add(tsk->stime, tsk->signal->stime),
tsk->se.sum_exec_runtime + tsk->signal->sum_sched_runtime);
}
/*
* Set the expiry times of all the threads in the process so one of them
* will go off before the process cumulative expiry total is reached.
*/
static void process_timer_rebalance(struct task_struct *p,
unsigned int clock_idx,
union cpu_time_count expires,
union cpu_time_count val)
{
cputime_t ticks, left;
unsigned long long ns, nsleft;
struct task_struct *t = p;
unsigned int nthreads = atomic_read(&p->signal->live);
if (!nthreads)
return;
switch (clock_idx) {
default:
BUG();
break;
case CPUCLOCK_PROF:
[PATCH] posix-cpu-timers: prevent signal delivery starvation The integer divisions in the timer accounting code can round the result down to 0. Adding 0 is without effect and the signal delivery stops. Clamp the division result to minimum 1 to avoid this. Problem was reported by Seongbae Park <spark@google.com>, who provided also an inital patch. Roland sayeth: I have had some more time to think about the problem, and to reproduce it using Toyo's test case. For the record, if my understanding of the problem is correct, this happens only in one very particular case. First, the expiry time has to be so soon that in cputime_t units (usually 1s/HZ ticks) it's < nthreads so the division yields zero. Second, it only affects each thread that is so new that its CPU time accumulation is zero so now+0 is still zero and ->it_*_expires winds up staying zero. For the VIRT and PROF clocks when cputime_t is tick granularity (or the SCHED clock on configurations where sched_clock's value only advances on clock ticks), this is not hard to arrange with new threads starting up and blocking before they accumulate a whole tick of CPU time. That's what happens in Toyo's test case. Note that in general it is fine for that division to round down to zero, and set each thread's expiry time to its "now" time. The problem only arises with thread's whose "now" value is still zero, so that now+0 winds up 0 and is interpreted as "not set" instead of ">= now". So it would be a sufficient and more precise fix to just use max(ticks, 1) inside the loop when setting each it_*_expires value. But, it does no harm to round the division up to one and always advance every thread's expiry time. If the thread didn't already fire timers for the expiry time of "now", there is no expectation that it will do so before the next tick anyway. So I followed Thomas's patch in lifting the max out of the loops. This patch also covers the reload cases, which are harder to write a test for (and I didn't try). I've tested it with Toyo's case and it fixes that. [toyoa@mvista.com: fix: min_t -> max_t] Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@elte.hu> Signed-off-by: Roland McGrath <roland@redhat.com> Cc: Daniel Walker <dwalker@mvista.com> Cc: Toyo Abe <toyoa@mvista.com> Cc: john stultz <johnstul@us.ibm.com> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: Seongbae Park <spark@google.com> Cc: Peter Mattis <pmattis@google.com> Cc: Rohit Seth <rohitseth@google.com> Cc: Martin Bligh <mbligh@google.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-17 09:09:39 +02:00
left = cputime_div_non_zero(cputime_sub(expires.cpu, val.cpu),
nthreads);
do {
if (likely(!(t->flags & PF_EXITING))) {
ticks = cputime_add(prof_ticks(t), left);
if (cputime_eq(t->it_prof_expires,
cputime_zero) ||
cputime_gt(t->it_prof_expires, ticks)) {
t->it_prof_expires = ticks;
}
}
t = next_thread(t);
} while (t != p);
break;
case CPUCLOCK_VIRT:
[PATCH] posix-cpu-timers: prevent signal delivery starvation The integer divisions in the timer accounting code can round the result down to 0. Adding 0 is without effect and the signal delivery stops. Clamp the division result to minimum 1 to avoid this. Problem was reported by Seongbae Park <spark@google.com>, who provided also an inital patch. Roland sayeth: I have had some more time to think about the problem, and to reproduce it using Toyo's test case. For the record, if my understanding of the problem is correct, this happens only in one very particular case. First, the expiry time has to be so soon that in cputime_t units (usually 1s/HZ ticks) it's < nthreads so the division yields zero. Second, it only affects each thread that is so new that its CPU time accumulation is zero so now+0 is still zero and ->it_*_expires winds up staying zero. For the VIRT and PROF clocks when cputime_t is tick granularity (or the SCHED clock on configurations where sched_clock's value only advances on clock ticks), this is not hard to arrange with new threads starting up and blocking before they accumulate a whole tick of CPU time. That's what happens in Toyo's test case. Note that in general it is fine for that division to round down to zero, and set each thread's expiry time to its "now" time. The problem only arises with thread's whose "now" value is still zero, so that now+0 winds up 0 and is interpreted as "not set" instead of ">= now". So it would be a sufficient and more precise fix to just use max(ticks, 1) inside the loop when setting each it_*_expires value. But, it does no harm to round the division up to one and always advance every thread's expiry time. If the thread didn't already fire timers for the expiry time of "now", there is no expectation that it will do so before the next tick anyway. So I followed Thomas's patch in lifting the max out of the loops. This patch also covers the reload cases, which are harder to write a test for (and I didn't try). I've tested it with Toyo's case and it fixes that. [toyoa@mvista.com: fix: min_t -> max_t] Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@elte.hu> Signed-off-by: Roland McGrath <roland@redhat.com> Cc: Daniel Walker <dwalker@mvista.com> Cc: Toyo Abe <toyoa@mvista.com> Cc: john stultz <johnstul@us.ibm.com> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: Seongbae Park <spark@google.com> Cc: Peter Mattis <pmattis@google.com> Cc: Rohit Seth <rohitseth@google.com> Cc: Martin Bligh <mbligh@google.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-17 09:09:39 +02:00
left = cputime_div_non_zero(cputime_sub(expires.cpu, val.cpu),
nthreads);
do {
if (likely(!(t->flags & PF_EXITING))) {
ticks = cputime_add(virt_ticks(t), left);
if (cputime_eq(t->it_virt_expires,
cputime_zero) ||
cputime_gt(t->it_virt_expires, ticks)) {
t->it_virt_expires = ticks;
}
}
t = next_thread(t);
} while (t != p);
break;
case CPUCLOCK_SCHED:
nsleft = expires.sched - val.sched;
do_div(nsleft, nthreads);
[PATCH] posix-cpu-timers: prevent signal delivery starvation The integer divisions in the timer accounting code can round the result down to 0. Adding 0 is without effect and the signal delivery stops. Clamp the division result to minimum 1 to avoid this. Problem was reported by Seongbae Park <spark@google.com>, who provided also an inital patch. Roland sayeth: I have had some more time to think about the problem, and to reproduce it using Toyo's test case. For the record, if my understanding of the problem is correct, this happens only in one very particular case. First, the expiry time has to be so soon that in cputime_t units (usually 1s/HZ ticks) it's < nthreads so the division yields zero. Second, it only affects each thread that is so new that its CPU time accumulation is zero so now+0 is still zero and ->it_*_expires winds up staying zero. For the VIRT and PROF clocks when cputime_t is tick granularity (or the SCHED clock on configurations where sched_clock's value only advances on clock ticks), this is not hard to arrange with new threads starting up and blocking before they accumulate a whole tick of CPU time. That's what happens in Toyo's test case. Note that in general it is fine for that division to round down to zero, and set each thread's expiry time to its "now" time. The problem only arises with thread's whose "now" value is still zero, so that now+0 winds up 0 and is interpreted as "not set" instead of ">= now". So it would be a sufficient and more precise fix to just use max(ticks, 1) inside the loop when setting each it_*_expires value. But, it does no harm to round the division up to one and always advance every thread's expiry time. If the thread didn't already fire timers for the expiry time of "now", there is no expectation that it will do so before the next tick anyway. So I followed Thomas's patch in lifting the max out of the loops. This patch also covers the reload cases, which are harder to write a test for (and I didn't try). I've tested it with Toyo's case and it fixes that. [toyoa@mvista.com: fix: min_t -> max_t] Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@elte.hu> Signed-off-by: Roland McGrath <roland@redhat.com> Cc: Daniel Walker <dwalker@mvista.com> Cc: Toyo Abe <toyoa@mvista.com> Cc: john stultz <johnstul@us.ibm.com> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: Seongbae Park <spark@google.com> Cc: Peter Mattis <pmattis@google.com> Cc: Rohit Seth <rohitseth@google.com> Cc: Martin Bligh <mbligh@google.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-17 09:09:39 +02:00
nsleft = max_t(unsigned long long, nsleft, 1);
do {
if (likely(!(t->flags & PF_EXITING))) {
ns = t->se.sum_exec_runtime + nsleft;
if (t->it_sched_expires == 0 ||
t->it_sched_expires > ns) {
t->it_sched_expires = ns;
}
}
t = next_thread(t);
} while (t != p);
break;
}
}
static void clear_dead_task(struct k_itimer *timer, union cpu_time_count now)
{
/*
* That's all for this thread or process.
* We leave our residual in expires to be reported.
*/
put_task_struct(timer->it.cpu.task);
timer->it.cpu.task = NULL;
timer->it.cpu.expires = cpu_time_sub(timer->it_clock,
timer->it.cpu.expires,
now);
}
/*
* Insert the timer on the appropriate list before any timers that
* expire later. This must be called with the tasklist_lock held
* for reading, and interrupts disabled.
*/
static void arm_timer(struct k_itimer *timer, union cpu_time_count now)
{
struct task_struct *p = timer->it.cpu.task;
struct list_head *head, *listpos;
struct cpu_timer_list *const nt = &timer->it.cpu;
struct cpu_timer_list *next;
unsigned long i;
head = (CPUCLOCK_PERTHREAD(timer->it_clock) ?
p->cpu_timers : p->signal->cpu_timers);
head += CPUCLOCK_WHICH(timer->it_clock);
BUG_ON(!irqs_disabled());
spin_lock(&p->sighand->siglock);
listpos = head;
if (CPUCLOCK_WHICH(timer->it_clock) == CPUCLOCK_SCHED) {
list_for_each_entry(next, head, entry) {
if (next->expires.sched > nt->expires.sched)
break;
listpos = &next->entry;
}
} else {
list_for_each_entry(next, head, entry) {
if (cputime_gt(next->expires.cpu, nt->expires.cpu))
break;
listpos = &next->entry;
}
}
list_add(&nt->entry, listpos);
if (listpos == head) {
/*
* We are the new earliest-expiring timer.
* If we are a thread timer, there can always
* be a process timer telling us to stop earlier.
*/
if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
switch (CPUCLOCK_WHICH(timer->it_clock)) {
default:
BUG();
case CPUCLOCK_PROF:
if (cputime_eq(p->it_prof_expires,
cputime_zero) ||
cputime_gt(p->it_prof_expires,
nt->expires.cpu))
p->it_prof_expires = nt->expires.cpu;
break;
case CPUCLOCK_VIRT:
if (cputime_eq(p->it_virt_expires,
cputime_zero) ||
cputime_gt(p->it_virt_expires,
nt->expires.cpu))
p->it_virt_expires = nt->expires.cpu;
break;
case CPUCLOCK_SCHED:
if (p->it_sched_expires == 0 ||
p->it_sched_expires > nt->expires.sched)
p->it_sched_expires = nt->expires.sched;
break;
}
} else {
/*
* For a process timer, we must balance
* all the live threads' expirations.
*/
switch (CPUCLOCK_WHICH(timer->it_clock)) {
default:
BUG();
case CPUCLOCK_VIRT:
if (!cputime_eq(p->signal->it_virt_expires,
cputime_zero) &&
cputime_lt(p->signal->it_virt_expires,
timer->it.cpu.expires.cpu))
break;
goto rebalance;
case CPUCLOCK_PROF:
if (!cputime_eq(p->signal->it_prof_expires,
cputime_zero) &&
cputime_lt(p->signal->it_prof_expires,
timer->it.cpu.expires.cpu))
break;
i = p->signal->rlim[RLIMIT_CPU].rlim_cur;
if (i != RLIM_INFINITY &&
i <= cputime_to_secs(timer->it.cpu.expires.cpu))
break;
goto rebalance;
case CPUCLOCK_SCHED:
rebalance:
process_timer_rebalance(
timer->it.cpu.task,
CPUCLOCK_WHICH(timer->it_clock),
timer->it.cpu.expires, now);
break;
}
}
}
spin_unlock(&p->sighand->siglock);
}
/*
* The timer is locked, fire it and arrange for its reload.
*/
static void cpu_timer_fire(struct k_itimer *timer)
{
if (unlikely(timer->sigq == NULL)) {
/*
* This a special case for clock_nanosleep,
* not a normal timer from sys_timer_create.
*/
wake_up_process(timer->it_process);
timer->it.cpu.expires.sched = 0;
} else if (timer->it.cpu.incr.sched == 0) {
/*
* One-shot timer. Clear it as soon as it's fired.
*/
posix_timer_event(timer, 0);
timer->it.cpu.expires.sched = 0;
} else if (posix_timer_event(timer, ++timer->it_requeue_pending)) {
/*
* The signal did not get queued because the signal
* was ignored, so we won't get any callback to
* reload the timer. But we need to keep it
* ticking in case the signal is deliverable next time.
*/
posix_cpu_timer_schedule(timer);
}
}
/*
* Guts of sys_timer_settime for CPU timers.
* This is called with the timer locked and interrupts disabled.
* If we return TIMER_RETRY, it's necessary to release the timer's lock
* and try again. (This happens when the timer is in the middle of firing.)
*/
int posix_cpu_timer_set(struct k_itimer *timer, int flags,
struct itimerspec *new, struct itimerspec *old)
{
struct task_struct *p = timer->it.cpu.task;
union cpu_time_count old_expires, new_expires, val;
int ret;
if (unlikely(p == NULL)) {
/*
* Timer refers to a dead task's clock.
*/
return -ESRCH;
}
new_expires = timespec_to_sample(timer->it_clock, &new->it_value);
read_lock(&tasklist_lock);
/*
* We need the tasklist_lock to protect against reaping that
* clears p->signal. If p has just been reaped, we can no
* longer get any information about it at all.
*/
if (unlikely(p->signal == NULL)) {
read_unlock(&tasklist_lock);
put_task_struct(p);
timer->it.cpu.task = NULL;
return -ESRCH;
}
/*
* Disarm any old timer after extracting its expiry time.
*/
BUG_ON(!irqs_disabled());
ret = 0;
spin_lock(&p->sighand->siglock);
old_expires = timer->it.cpu.expires;
if (unlikely(timer->it.cpu.firing)) {
timer->it.cpu.firing = -1;
ret = TIMER_RETRY;
} else
list_del_init(&timer->it.cpu.entry);
spin_unlock(&p->sighand->siglock);
/*
* We need to sample the current value to convert the new
* value from to relative and absolute, and to convert the
* old value from absolute to relative. To set a process
* timer, we need a sample to balance the thread expiry
* times (in arm_timer). With an absolute time, we must
* check if it's already passed. In short, we need a sample.
*/
if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
cpu_clock_sample(timer->it_clock, p, &val);
} else {
cpu_clock_sample_group(timer->it_clock, p, &val);
}
if (old) {
if (old_expires.sched == 0) {
old->it_value.tv_sec = 0;
old->it_value.tv_nsec = 0;
} else {
/*
* Update the timer in case it has
* overrun already. If it has,
* we'll report it as having overrun
* and with the next reloaded timer
* already ticking, though we are
* swallowing that pending
* notification here to install the
* new setting.
*/
bump_cpu_timer(timer, val);
if (cpu_time_before(timer->it_clock, val,
timer->it.cpu.expires)) {
old_expires = cpu_time_sub(
timer->it_clock,
timer->it.cpu.expires, val);
sample_to_timespec(timer->it_clock,
old_expires,
&old->it_value);
} else {
old->it_value.tv_nsec = 1;
old->it_value.tv_sec = 0;
}
}
}
if (unlikely(ret)) {
/*
* We are colliding with the timer actually firing.
* Punt after filling in the timer's old value, and
* disable this firing since we are already reporting
* it as an overrun (thanks to bump_cpu_timer above).
*/
read_unlock(&tasklist_lock);
goto out;
}
if (new_expires.sched != 0 && !(flags & TIMER_ABSTIME)) {
cpu_time_add(timer->it_clock, &new_expires, val);
}
/*
* Install the new expiry time (or zero).
* For a timer with no notification action, we don't actually
* arm the timer (we'll just fake it for timer_gettime).
*/
timer->it.cpu.expires = new_expires;
if (new_expires.sched != 0 &&
(timer->it_sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_NONE &&
cpu_time_before(timer->it_clock, val, new_expires)) {
arm_timer(timer, val);
}
read_unlock(&tasklist_lock);
/*
* Install the new reload setting, and
* set up the signal and overrun bookkeeping.
*/
timer->it.cpu.incr = timespec_to_sample(timer->it_clock,
&new->it_interval);
/*
* This acts as a modification timestamp for the timer,
* so any automatic reload attempt will punt on seeing
* that we have reset the timer manually.
*/
timer->it_requeue_pending = (timer->it_requeue_pending + 2) &
~REQUEUE_PENDING;
timer->it_overrun_last = 0;
timer->it_overrun = -1;
if (new_expires.sched != 0 &&
(timer->it_sigev_notify & ~SIGEV_THREAD_ID) != SIGEV_NONE &&
!cpu_time_before(timer->it_clock, val, new_expires)) {
/*
* The designated time already passed, so we notify
* immediately, even if the thread never runs to
* accumulate more time on this clock.
*/
cpu_timer_fire(timer);
}
ret = 0;
out:
if (old) {
sample_to_timespec(timer->it_clock,
timer->it.cpu.incr, &old->it_interval);
}
return ret;
}
void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
{
union cpu_time_count now;
struct task_struct *p = timer->it.cpu.task;
int clear_dead;
/*
* Easy part: convert the reload time.
*/
sample_to_timespec(timer->it_clock,
timer->it.cpu.incr, &itp->it_interval);
if (timer->it.cpu.expires.sched == 0) { /* Timer not armed at all. */
itp->it_value.tv_sec = itp->it_value.tv_nsec = 0;
return;
}
if (unlikely(p == NULL)) {
/*
* This task already died and the timer will never fire.
* In this case, expires is actually the dead value.
*/
dead:
sample_to_timespec(timer->it_clock, timer->it.cpu.expires,
&itp->it_value);
return;
}
/*
* Sample the clock to take the difference with the expiry time.
*/
if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
cpu_clock_sample(timer->it_clock, p, &now);
clear_dead = p->exit_state;
} else {
read_lock(&tasklist_lock);
if (unlikely(p->signal == NULL)) {
/*
* The process has been reaped.
* We can't even collect a sample any more.
* Call the timer disarmed, nothing else to do.
*/
put_task_struct(p);
timer->it.cpu.task = NULL;
timer->it.cpu.expires.sched = 0;
read_unlock(&tasklist_lock);
goto dead;
} else {
cpu_clock_sample_group(timer->it_clock, p, &now);
clear_dead = (unlikely(p->exit_state) &&
thread_group_empty(p));
}
read_unlock(&tasklist_lock);
}
if ((timer->it_sigev_notify & ~SIGEV_THREAD_ID) == SIGEV_NONE) {
if (timer->it.cpu.incr.sched == 0 &&
cpu_time_before(timer->it_clock,
timer->it.cpu.expires, now)) {
/*
* Do-nothing timer expired and has no reload,
* so it's as if it was never set.
*/
timer->it.cpu.expires.sched = 0;
itp->it_value.tv_sec = itp->it_value.tv_nsec = 0;
return;
}
/*
* Account for any expirations and reloads that should
* have happened.
*/
bump_cpu_timer(timer, now);
}
if (unlikely(clear_dead)) {
/*
* We've noticed that the thread is dead, but
* not yet reaped. Take this opportunity to
* drop our task ref.
*/
clear_dead_task(timer, now);
goto dead;
}
if (cpu_time_before(timer->it_clock, now, timer->it.cpu.expires)) {
sample_to_timespec(timer->it_clock,
cpu_time_sub(timer->it_clock,
timer->it.cpu.expires, now),
&itp->it_value);
} else {
/*
* The timer should have expired already, but the firing
* hasn't taken place yet. Say it's just about to expire.
*/
itp->it_value.tv_nsec = 1;
itp->it_value.tv_sec = 0;
}
}
/*
* Check for any per-thread CPU timers that have fired and move them off
* the tsk->cpu_timers[N] list onto the firing list. Here we update the
* tsk->it_*_expires values to reflect the remaining thread CPU timers.
*/
static void check_thread_timers(struct task_struct *tsk,
struct list_head *firing)
{
int maxfire;
struct list_head *timers = tsk->cpu_timers;
struct signal_struct *const sig = tsk->signal;
maxfire = 20;
tsk->it_prof_expires = cputime_zero;
while (!list_empty(timers)) {
struct cpu_timer_list *t = list_first_entry(timers,
struct cpu_timer_list,
entry);
if (!--maxfire || cputime_lt(prof_ticks(tsk), t->expires.cpu)) {
tsk->it_prof_expires = t->expires.cpu;
break;
}
t->firing = 1;
list_move_tail(&t->entry, firing);
}
++timers;
maxfire = 20;
tsk->it_virt_expires = cputime_zero;
while (!list_empty(timers)) {
struct cpu_timer_list *t = list_first_entry(timers,
struct cpu_timer_list,
entry);
if (!--maxfire || cputime_lt(virt_ticks(tsk), t->expires.cpu)) {
tsk->it_virt_expires = t->expires.cpu;
break;
}
t->firing = 1;
list_move_tail(&t->entry, firing);
}
++timers;
maxfire = 20;
tsk->it_sched_expires = 0;
while (!list_empty(timers)) {
struct cpu_timer_list *t = list_first_entry(timers,
struct cpu_timer_list,
entry);
if (!--maxfire || tsk->se.sum_exec_runtime < t->expires.sched) {
tsk->it_sched_expires = t->expires.sched;
break;
}
t->firing = 1;
list_move_tail(&t->entry, firing);
}
/*
* Check for the special case thread timers.
*/
if (sig->rlim[RLIMIT_RTTIME].rlim_cur != RLIM_INFINITY) {
unsigned long hard = sig->rlim[RLIMIT_RTTIME].rlim_max;
unsigned long *soft = &sig->rlim[RLIMIT_RTTIME].rlim_cur;
if (hard != RLIM_INFINITY &&
tsk->rt.timeout > DIV_ROUND_UP(hard, USEC_PER_SEC/HZ)) {
/*
* At the hard limit, we just die.
* No need to calculate anything else now.
*/
__group_send_sig_info(SIGKILL, SEND_SIG_PRIV, tsk);
return;
}
if (tsk->rt.timeout > DIV_ROUND_UP(*soft, USEC_PER_SEC/HZ)) {
/*
* At the soft limit, send a SIGXCPU every second.
*/
if (sig->rlim[RLIMIT_RTTIME].rlim_cur
< sig->rlim[RLIMIT_RTTIME].rlim_max) {
sig->rlim[RLIMIT_RTTIME].rlim_cur +=
USEC_PER_SEC;
}
printk(KERN_INFO
"RT Watchdog Timeout: %s[%d]\n",
tsk->comm, task_pid_nr(tsk));
__group_send_sig_info(SIGXCPU, SEND_SIG_PRIV, tsk);
}
}
}
/*
* Check for any per-thread CPU timers that have fired and move them
* off the tsk->*_timers list onto the firing list. Per-thread timers
* have already been taken off.
*/
static void check_process_timers(struct task_struct *tsk,
struct list_head *firing)
{
int maxfire;
struct signal_struct *const sig = tsk->signal;
cputime_t utime, stime, ptime, virt_expires, prof_expires;
unsigned long long sum_sched_runtime, sched_expires;
struct task_struct *t;
struct list_head *timers = sig->cpu_timers;
/*
* Don't sample the current process CPU clocks if there are no timers.
*/
if (list_empty(&timers[CPUCLOCK_PROF]) &&
cputime_eq(sig->it_prof_expires, cputime_zero) &&
sig->rlim[RLIMIT_CPU].rlim_cur == RLIM_INFINITY &&
list_empty(&timers[CPUCLOCK_VIRT]) &&
cputime_eq(sig->it_virt_expires, cputime_zero) &&
list_empty(&timers[CPUCLOCK_SCHED]))
return;
/*
* Collect the current process totals.
*/
utime = sig->utime;
stime = sig->stime;
sum_sched_runtime = sig->sum_sched_runtime;
t = tsk;
do {
utime = cputime_add(utime, t->utime);
stime = cputime_add(stime, t->stime);
sum_sched_runtime += t->se.sum_exec_runtime;
t = next_thread(t);
} while (t != tsk);
ptime = cputime_add(utime, stime);
maxfire = 20;
prof_expires = cputime_zero;
while (!list_empty(timers)) {
struct cpu_timer_list *tl = list_first_entry(timers,
struct cpu_timer_list,
entry);
if (!--maxfire || cputime_lt(ptime, tl->expires.cpu)) {
prof_expires = tl->expires.cpu;
break;
}
tl->firing = 1;
list_move_tail(&tl->entry, firing);
}
++timers;
maxfire = 20;
virt_expires = cputime_zero;
while (!list_empty(timers)) {
struct cpu_timer_list *tl = list_first_entry(timers,
struct cpu_timer_list,
entry);
if (!--maxfire || cputime_lt(utime, tl->expires.cpu)) {
virt_expires = tl->expires.cpu;
break;
}
tl->firing = 1;
list_move_tail(&tl->entry, firing);
}
++timers;
maxfire = 20;
sched_expires = 0;
while (!list_empty(timers)) {
struct cpu_timer_list *tl = list_first_entry(timers,
struct cpu_timer_list,
entry);
if (!--maxfire || sum_sched_runtime < tl->expires.sched) {
sched_expires = tl->expires.sched;
break;
}
tl->firing = 1;
list_move_tail(&tl->entry, firing);
}
/*
* Check for the special case process timers.
*/
if (!cputime_eq(sig->it_prof_expires, cputime_zero)) {
if (cputime_ge(ptime, sig->it_prof_expires)) {
/* ITIMER_PROF fires and reloads. */
sig->it_prof_expires = sig->it_prof_incr;
if (!cputime_eq(sig->it_prof_expires, cputime_zero)) {
sig->it_prof_expires = cputime_add(
sig->it_prof_expires, ptime);
}
__group_send_sig_info(SIGPROF, SEND_SIG_PRIV, tsk);
}
if (!cputime_eq(sig->it_prof_expires, cputime_zero) &&
(cputime_eq(prof_expires, cputime_zero) ||
cputime_lt(sig->it_prof_expires, prof_expires))) {
prof_expires = sig->it_prof_expires;
}
}
if (!cputime_eq(sig->it_virt_expires, cputime_zero)) {
if (cputime_ge(utime, sig->it_virt_expires)) {
/* ITIMER_VIRTUAL fires and reloads. */
sig->it_virt_expires = sig->it_virt_incr;
if (!cputime_eq(sig->it_virt_expires, cputime_zero)) {
sig->it_virt_expires = cputime_add(
sig->it_virt_expires, utime);
}
__group_send_sig_info(SIGVTALRM, SEND_SIG_PRIV, tsk);
}
if (!cputime_eq(sig->it_virt_expires, cputime_zero) &&
(cputime_eq(virt_expires, cputime_zero) ||
cputime_lt(sig->it_virt_expires, virt_expires))) {
virt_expires = sig->it_virt_expires;
}
}
if (sig->rlim[RLIMIT_CPU].rlim_cur != RLIM_INFINITY) {
unsigned long psecs = cputime_to_secs(ptime);
cputime_t x;
if (psecs >= sig->rlim[RLIMIT_CPU].rlim_max) {
/*
* At the hard limit, we just die.
* No need to calculate anything else now.
*/
__group_send_sig_info(SIGKILL, SEND_SIG_PRIV, tsk);
return;
}
if (psecs >= sig->rlim[RLIMIT_CPU].rlim_cur) {
/*
* At the soft limit, send a SIGXCPU every second.
*/
__group_send_sig_info(SIGXCPU, SEND_SIG_PRIV, tsk);
if (sig->rlim[RLIMIT_CPU].rlim_cur
< sig->rlim[RLIMIT_CPU].rlim_max) {
sig->rlim[RLIMIT_CPU].rlim_cur++;
}
}
x = secs_to_cputime(sig->rlim[RLIMIT_CPU].rlim_cur);
if (cputime_eq(prof_expires, cputime_zero) ||
cputime_lt(x, prof_expires)) {
prof_expires = x;
}
}
if (!cputime_eq(prof_expires, cputime_zero) ||
!cputime_eq(virt_expires, cputime_zero) ||
sched_expires != 0) {
/*
* Rebalance the threads' expiry times for the remaining
* process CPU timers.
*/
cputime_t prof_left, virt_left, ticks;
unsigned long long sched_left, sched;
const unsigned int nthreads = atomic_read(&sig->live);
if (!nthreads)
return;
prof_left = cputime_sub(prof_expires, utime);
prof_left = cputime_sub(prof_left, stime);
[PATCH] posix-cpu-timers: prevent signal delivery starvation The integer divisions in the timer accounting code can round the result down to 0. Adding 0 is without effect and the signal delivery stops. Clamp the division result to minimum 1 to avoid this. Problem was reported by Seongbae Park <spark@google.com>, who provided also an inital patch. Roland sayeth: I have had some more time to think about the problem, and to reproduce it using Toyo's test case. For the record, if my understanding of the problem is correct, this happens only in one very particular case. First, the expiry time has to be so soon that in cputime_t units (usually 1s/HZ ticks) it's < nthreads so the division yields zero. Second, it only affects each thread that is so new that its CPU time accumulation is zero so now+0 is still zero and ->it_*_expires winds up staying zero. For the VIRT and PROF clocks when cputime_t is tick granularity (or the SCHED clock on configurations where sched_clock's value only advances on clock ticks), this is not hard to arrange with new threads starting up and blocking before they accumulate a whole tick of CPU time. That's what happens in Toyo's test case. Note that in general it is fine for that division to round down to zero, and set each thread's expiry time to its "now" time. The problem only arises with thread's whose "now" value is still zero, so that now+0 winds up 0 and is interpreted as "not set" instead of ">= now". So it would be a sufficient and more precise fix to just use max(ticks, 1) inside the loop when setting each it_*_expires value. But, it does no harm to round the division up to one and always advance every thread's expiry time. If the thread didn't already fire timers for the expiry time of "now", there is no expectation that it will do so before the next tick anyway. So I followed Thomas's patch in lifting the max out of the loops. This patch also covers the reload cases, which are harder to write a test for (and I didn't try). I've tested it with Toyo's case and it fixes that. [toyoa@mvista.com: fix: min_t -> max_t] Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@elte.hu> Signed-off-by: Roland McGrath <roland@redhat.com> Cc: Daniel Walker <dwalker@mvista.com> Cc: Toyo Abe <toyoa@mvista.com> Cc: john stultz <johnstul@us.ibm.com> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: Seongbae Park <spark@google.com> Cc: Peter Mattis <pmattis@google.com> Cc: Rohit Seth <rohitseth@google.com> Cc: Martin Bligh <mbligh@google.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-17 09:09:39 +02:00
prof_left = cputime_div_non_zero(prof_left, nthreads);
virt_left = cputime_sub(virt_expires, utime);
[PATCH] posix-cpu-timers: prevent signal delivery starvation The integer divisions in the timer accounting code can round the result down to 0. Adding 0 is without effect and the signal delivery stops. Clamp the division result to minimum 1 to avoid this. Problem was reported by Seongbae Park <spark@google.com>, who provided also an inital patch. Roland sayeth: I have had some more time to think about the problem, and to reproduce it using Toyo's test case. For the record, if my understanding of the problem is correct, this happens only in one very particular case. First, the expiry time has to be so soon that in cputime_t units (usually 1s/HZ ticks) it's < nthreads so the division yields zero. Second, it only affects each thread that is so new that its CPU time accumulation is zero so now+0 is still zero and ->it_*_expires winds up staying zero. For the VIRT and PROF clocks when cputime_t is tick granularity (or the SCHED clock on configurations where sched_clock's value only advances on clock ticks), this is not hard to arrange with new threads starting up and blocking before they accumulate a whole tick of CPU time. That's what happens in Toyo's test case. Note that in general it is fine for that division to round down to zero, and set each thread's expiry time to its "now" time. The problem only arises with thread's whose "now" value is still zero, so that now+0 winds up 0 and is interpreted as "not set" instead of ">= now". So it would be a sufficient and more precise fix to just use max(ticks, 1) inside the loop when setting each it_*_expires value. But, it does no harm to round the division up to one and always advance every thread's expiry time. If the thread didn't already fire timers for the expiry time of "now", there is no expectation that it will do so before the next tick anyway. So I followed Thomas's patch in lifting the max out of the loops. This patch also covers the reload cases, which are harder to write a test for (and I didn't try). I've tested it with Toyo's case and it fixes that. [toyoa@mvista.com: fix: min_t -> max_t] Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@elte.hu> Signed-off-by: Roland McGrath <roland@redhat.com> Cc: Daniel Walker <dwalker@mvista.com> Cc: Toyo Abe <toyoa@mvista.com> Cc: john stultz <johnstul@us.ibm.com> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: Seongbae Park <spark@google.com> Cc: Peter Mattis <pmattis@google.com> Cc: Rohit Seth <rohitseth@google.com> Cc: Martin Bligh <mbligh@google.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-17 09:09:39 +02:00
virt_left = cputime_div_non_zero(virt_left, nthreads);
if (sched_expires) {
sched_left = sched_expires - sum_sched_runtime;
do_div(sched_left, nthreads);
[PATCH] posix-cpu-timers: prevent signal delivery starvation The integer divisions in the timer accounting code can round the result down to 0. Adding 0 is without effect and the signal delivery stops. Clamp the division result to minimum 1 to avoid this. Problem was reported by Seongbae Park <spark@google.com>, who provided also an inital patch. Roland sayeth: I have had some more time to think about the problem, and to reproduce it using Toyo's test case. For the record, if my understanding of the problem is correct, this happens only in one very particular case. First, the expiry time has to be so soon that in cputime_t units (usually 1s/HZ ticks) it's < nthreads so the division yields zero. Second, it only affects each thread that is so new that its CPU time accumulation is zero so now+0 is still zero and ->it_*_expires winds up staying zero. For the VIRT and PROF clocks when cputime_t is tick granularity (or the SCHED clock on configurations where sched_clock's value only advances on clock ticks), this is not hard to arrange with new threads starting up and blocking before they accumulate a whole tick of CPU time. That's what happens in Toyo's test case. Note that in general it is fine for that division to round down to zero, and set each thread's expiry time to its "now" time. The problem only arises with thread's whose "now" value is still zero, so that now+0 winds up 0 and is interpreted as "not set" instead of ">= now". So it would be a sufficient and more precise fix to just use max(ticks, 1) inside the loop when setting each it_*_expires value. But, it does no harm to round the division up to one and always advance every thread's expiry time. If the thread didn't already fire timers for the expiry time of "now", there is no expectation that it will do so before the next tick anyway. So I followed Thomas's patch in lifting the max out of the loops. This patch also covers the reload cases, which are harder to write a test for (and I didn't try). I've tested it with Toyo's case and it fixes that. [toyoa@mvista.com: fix: min_t -> max_t] Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@elte.hu> Signed-off-by: Roland McGrath <roland@redhat.com> Cc: Daniel Walker <dwalker@mvista.com> Cc: Toyo Abe <toyoa@mvista.com> Cc: john stultz <johnstul@us.ibm.com> Cc: Roman Zippel <zippel@linux-m68k.org> Cc: Seongbae Park <spark@google.com> Cc: Peter Mattis <pmattis@google.com> Cc: Rohit Seth <rohitseth@google.com> Cc: Martin Bligh <mbligh@google.com> Cc: <stable@kernel.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2006-10-17 09:09:39 +02:00
sched_left = max_t(unsigned long long, sched_left, 1);
} else {
sched_left = 0;
}
t = tsk;
do {
if (unlikely(t->flags & PF_EXITING))
continue;
ticks = cputime_add(cputime_add(t->utime, t->stime),
prof_left);
if (!cputime_eq(prof_expires, cputime_zero) &&
(cputime_eq(t->it_prof_expires, cputime_zero) ||
cputime_gt(t->it_prof_expires, ticks))) {
t->it_prof_expires = ticks;
}
ticks = cputime_add(t->utime, virt_left);
if (!cputime_eq(virt_expires, cputime_zero) &&
(cputime_eq(t->it_virt_expires, cputime_zero) ||
cputime_gt(t->it_virt_expires, ticks))) {
t->it_virt_expires = ticks;
}
sched = t->se.sum_exec_runtime + sched_left;
if (sched_expires && (t->it_sched_expires == 0 ||
t->it_sched_expires > sched)) {
t->it_sched_expires = sched;
}
} while ((t = next_thread(t)) != tsk);
}
}
/*
* This is called from the signal code (via do_schedule_next_timer)
* when the last timer signal was delivered and we have to reload the timer.
*/
void posix_cpu_timer_schedule(struct k_itimer *timer)
{
struct task_struct *p = timer->it.cpu.task;
union cpu_time_count now;
if (unlikely(p == NULL))
/*
* The task was cleaned up already, no future firings.
*/
goto out;
/*
* Fetch the current sample and update the timer's expiry time.
*/
if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
cpu_clock_sample(timer->it_clock, p, &now);
bump_cpu_timer(timer, now);
if (unlikely(p->exit_state)) {
clear_dead_task(timer, now);
goto out;
}
read_lock(&tasklist_lock); /* arm_timer needs it. */
} else {
read_lock(&tasklist_lock);
if (unlikely(p->signal == NULL)) {
/*
* The process has been reaped.
* We can't even collect a sample any more.
*/
put_task_struct(p);
timer->it.cpu.task = p = NULL;
timer->it.cpu.expires.sched = 0;
goto out_unlock;
} else if (unlikely(p->exit_state) && thread_group_empty(p)) {
/*
* We've noticed that the thread is dead, but
* not yet reaped. Take this opportunity to
* drop our task ref.
*/
clear_dead_task(timer, now);
goto out_unlock;
}
cpu_clock_sample_group(timer->it_clock, p, &now);
bump_cpu_timer(timer, now);
/* Leave the tasklist_lock locked for the call below. */
}
/*
* Now re-arm for the new expiry time.
*/
arm_timer(timer, now);
out_unlock:
read_unlock(&tasklist_lock);
out:
timer->it_overrun_last = timer->it_overrun;
timer->it_overrun = -1;
++timer->it_requeue_pending;
}
/*
* This is called from the timer interrupt handler. The irq handler has
* already updated our counts. We need to check if any timers fire now.
* Interrupts are disabled.
*/
void run_posix_cpu_timers(struct task_struct *tsk)
{
LIST_HEAD(firing);
struct k_itimer *timer, *next;
BUG_ON(!irqs_disabled());
#define UNEXPIRED(clock) \
(cputime_eq(tsk->it_##clock##_expires, cputime_zero) || \
cputime_lt(clock##_ticks(tsk), tsk->it_##clock##_expires))
if (UNEXPIRED(prof) && UNEXPIRED(virt) &&
(tsk->it_sched_expires == 0 ||
tsk->se.sum_exec_runtime < tsk->it_sched_expires))
return;
#undef UNEXPIRED
/*
* Double-check with locks held.
*/
read_lock(&tasklist_lock);
if (likely(tsk->signal != NULL)) {
spin_lock(&tsk->sighand->siglock);
/*
* Here we take off tsk->cpu_timers[N] and tsk->signal->cpu_timers[N]
* all the timers that are firing, and put them on the firing list.
*/
check_thread_timers(tsk, &firing);
check_process_timers(tsk, &firing);
/*
* We must release these locks before taking any timer's lock.
* There is a potential race with timer deletion here, as the
* siglock now protects our private firing list. We have set
* the firing flag in each timer, so that a deletion attempt
* that gets the timer lock before we do will give it up and
* spin until we've taken care of that timer below.
*/
spin_unlock(&tsk->sighand->siglock);
}
read_unlock(&tasklist_lock);
/*
* Now that all the timers on our list have the firing flag,
* noone will touch their list entries but us. We'll take
* each timer's lock before clearing its firing flag, so no
* timer call will interfere.
*/
list_for_each_entry_safe(timer, next, &firing, it.cpu.entry) {
int firing;
spin_lock(&timer->it_lock);
list_del_init(&timer->it.cpu.entry);
firing = timer->it.cpu.firing;
timer->it.cpu.firing = 0;
/*
* The firing flag is -1 if we collided with a reset
* of the timer, which already reported this
* almost-firing as an overrun. So don't generate an event.
*/
if (likely(firing >= 0)) {
cpu_timer_fire(timer);
}
spin_unlock(&timer->it_lock);
}
}
/*
* Set one of the process-wide special case CPU timers.
* The tasklist_lock and tsk->sighand->siglock must be held by the caller.
* The oldval argument is null for the RLIMIT_CPU timer, where *newval is
* absolute; non-null for ITIMER_*, where *newval is relative and we update
* it to be absolute, *oldval is absolute and we update it to be relative.
*/
void set_process_cpu_timer(struct task_struct *tsk, unsigned int clock_idx,
cputime_t *newval, cputime_t *oldval)
{
union cpu_time_count now;
struct list_head *head;
BUG_ON(clock_idx == CPUCLOCK_SCHED);
cpu_clock_sample_group_locked(clock_idx, tsk, &now);
if (oldval) {
if (!cputime_eq(*oldval, cputime_zero)) {
if (cputime_le(*oldval, now.cpu)) {
/* Just about to fire. */
*oldval = jiffies_to_cputime(1);
} else {
*oldval = cputime_sub(*oldval, now.cpu);
}
}
if (cputime_eq(*newval, cputime_zero))
return;
*newval = cputime_add(*newval, now.cpu);
/*
* If the RLIMIT_CPU timer will expire before the
* ITIMER_PROF timer, we have nothing else to do.
*/
if (tsk->signal->rlim[RLIMIT_CPU].rlim_cur
< cputime_to_secs(*newval))
return;
}
/*
* Check whether there are any process timers already set to fire
* before this one. If so, we don't have anything more to do.
*/
head = &tsk->signal->cpu_timers[clock_idx];
if (list_empty(head) ||
cputime_ge(list_first_entry(head,
struct cpu_timer_list, entry)->expires.cpu,
*newval)) {
/*
* Rejigger each thread's expiry time so that one will
* notice before we hit the process-cumulative expiry time.
*/
union cpu_time_count expires = { .sched = 0 };
expires.cpu = *newval;
process_timer_rebalance(tsk, clock_idx, expires, now);
}
}
static int do_cpu_nanosleep(const clockid_t which_clock, int flags,
struct timespec *rqtp, struct itimerspec *it)
{
struct k_itimer timer;
int error;
/*
* Set up a temporary timer and then wait for it to go off.
*/
memset(&timer, 0, sizeof timer);
spin_lock_init(&timer.it_lock);
timer.it_clock = which_clock;
timer.it_overrun = -1;
error = posix_cpu_timer_create(&timer);
timer.it_process = current;
if (!error) {
static struct itimerspec zero_it;
memset(it, 0, sizeof *it);
it->it_value = *rqtp;
spin_lock_irq(&timer.it_lock);
error = posix_cpu_timer_set(&timer, flags, it, NULL);
if (error) {
spin_unlock_irq(&timer.it_lock);
return error;
}
while (!signal_pending(current)) {
if (timer.it.cpu.expires.sched == 0) {
/*
* Our timer fired and was reset.
*/
spin_unlock_irq(&timer.it_lock);
return 0;
}
/*
* Block until cpu_timer_fire (or a signal) wakes us.
*/
__set_current_state(TASK_INTERRUPTIBLE);
spin_unlock_irq(&timer.it_lock);
schedule();
spin_lock_irq(&timer.it_lock);
}
/*
* We were interrupted by a signal.
*/
sample_to_timespec(which_clock, timer.it.cpu.expires, rqtp);
posix_cpu_timer_set(&timer, 0, &zero_it, it);
spin_unlock_irq(&timer.it_lock);
if ((it->it_value.tv_sec | it->it_value.tv_nsec) == 0) {
/*
* It actually did fire already.
*/
return 0;
}
error = -ERESTART_RESTARTBLOCK;
}
return error;
}
int posix_cpu_nsleep(const clockid_t which_clock, int flags,
struct timespec *rqtp, struct timespec __user *rmtp)
{
struct restart_block *restart_block =
&current_thread_info()->restart_block;
struct itimerspec it;
int error;
/*
* Diagnose required errors first.
*/
if (CPUCLOCK_PERTHREAD(which_clock) &&
(CPUCLOCK_PID(which_clock) == 0 ||
CPUCLOCK_PID(which_clock) == current->pid))
return -EINVAL;
error = do_cpu_nanosleep(which_clock, flags, rqtp, &it);
if (error == -ERESTART_RESTARTBLOCK) {
if (flags & TIMER_ABSTIME)
return -ERESTARTNOHAND;
/*
* Report back to the user the time still remaining.
*/
if (rmtp != NULL && copy_to_user(rmtp, &it.it_value, sizeof *rmtp))
return -EFAULT;
restart_block->fn = posix_cpu_nsleep_restart;
restart_block->arg0 = which_clock;
restart_block->arg1 = (unsigned long) rmtp;
restart_block->arg2 = rqtp->tv_sec;
restart_block->arg3 = rqtp->tv_nsec;
}
return error;
}
long posix_cpu_nsleep_restart(struct restart_block *restart_block)
{
clockid_t which_clock = restart_block->arg0;
struct timespec __user *rmtp;
struct timespec t;
struct itimerspec it;
int error;
rmtp = (struct timespec __user *) restart_block->arg1;
t.tv_sec = restart_block->arg2;
t.tv_nsec = restart_block->arg3;
restart_block->fn = do_no_restart_syscall;
error = do_cpu_nanosleep(which_clock, TIMER_ABSTIME, &t, &it);
if (error == -ERESTART_RESTARTBLOCK) {
/*
* Report back to the user the time still remaining.
*/
if (rmtp != NULL && copy_to_user(rmtp, &it.it_value, sizeof *rmtp))
return -EFAULT;
restart_block->fn = posix_cpu_nsleep_restart;
restart_block->arg0 = which_clock;
restart_block->arg1 = (unsigned long) rmtp;
restart_block->arg2 = t.tv_sec;
restart_block->arg3 = t.tv_nsec;
}
return error;
}
#define PROCESS_CLOCK MAKE_PROCESS_CPUCLOCK(0, CPUCLOCK_SCHED)
#define THREAD_CLOCK MAKE_THREAD_CPUCLOCK(0, CPUCLOCK_SCHED)
static int process_cpu_clock_getres(const clockid_t which_clock,
struct timespec *tp)
{
return posix_cpu_clock_getres(PROCESS_CLOCK, tp);
}
static int process_cpu_clock_get(const clockid_t which_clock,
struct timespec *tp)
{
return posix_cpu_clock_get(PROCESS_CLOCK, tp);
}
static int process_cpu_timer_create(struct k_itimer *timer)
{
timer->it_clock = PROCESS_CLOCK;
return posix_cpu_timer_create(timer);
}
static int process_cpu_nsleep(const clockid_t which_clock, int flags,
struct timespec *rqtp,
struct timespec __user *rmtp)
{
return posix_cpu_nsleep(PROCESS_CLOCK, flags, rqtp, rmtp);
}
static long process_cpu_nsleep_restart(struct restart_block *restart_block)
{
return -EINVAL;
}
static int thread_cpu_clock_getres(const clockid_t which_clock,
struct timespec *tp)
{
return posix_cpu_clock_getres(THREAD_CLOCK, tp);
}
static int thread_cpu_clock_get(const clockid_t which_clock,
struct timespec *tp)
{
return posix_cpu_clock_get(THREAD_CLOCK, tp);
}
static int thread_cpu_timer_create(struct k_itimer *timer)
{
timer->it_clock = THREAD_CLOCK;
return posix_cpu_timer_create(timer);
}
static int thread_cpu_nsleep(const clockid_t which_clock, int flags,
struct timespec *rqtp, struct timespec __user *rmtp)
{
return -EINVAL;
}
static long thread_cpu_nsleep_restart(struct restart_block *restart_block)
{
return -EINVAL;
}
static __init int init_posix_cpu_timers(void)
{
struct k_clock process = {
.clock_getres = process_cpu_clock_getres,
.clock_get = process_cpu_clock_get,
.clock_set = do_posix_clock_nosettime,
.timer_create = process_cpu_timer_create,
.nsleep = process_cpu_nsleep,
.nsleep_restart = process_cpu_nsleep_restart,
};
struct k_clock thread = {
.clock_getres = thread_cpu_clock_getres,
.clock_get = thread_cpu_clock_get,
.clock_set = do_posix_clock_nosettime,
.timer_create = thread_cpu_timer_create,
.nsleep = thread_cpu_nsleep,
.nsleep_restart = thread_cpu_nsleep_restart,
};
register_posix_clock(CLOCK_PROCESS_CPUTIME_ID, &process);
register_posix_clock(CLOCK_THREAD_CPUTIME_ID, &thread);
return 0;
}
__initcall(init_posix_cpu_timers);