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rcu: Exclude near-simultaneous RCU CPU stall warnings 

There is a two-jiffy delay between the time that a CPU will self-report
an RCU CPU stall warning and the time that some other CPU will report a
warning on behalf of the first CPU.  This has worked well in the past,
but on busy systems, it is possible for the two warnings to overlap,
which makes interpreting them extremely difficult.

This commit therefore uses a cmpxchg-based timing decision that
allows only one report in a given one-minute period (assuming default
stall-warning Kconfig parameters).  This approach will of course fail
if you are seeing minute-long vCPU preemption, but in that case the
overlapping RCU CPU stall warnings are the least of your worries.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2018-04-09 11:04:46 -07:00
parent ce11fae8d4
commit 8c42b1f39f
1 changed files with 11 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

26
kernel/rcu/tree.c
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@ -1368,7 +1368,6 @@ static inline void panic_on_rcu_stall(void)
static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum) static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
{ {
int cpu; int cpu;
long delta;
unsigned long flags; unsigned long flags;
unsigned long gpa; unsigned long gpa;
unsigned long j; unsigned long j;
@ -1381,18 +1380,6 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
if (rcu_cpu_stall_suppress) if (rcu_cpu_stall_suppress)
return; return;
/* Only let one CPU complain about others per time interval. */
raw_spin_lock_irqsave_rcu_node(rnp, flags);
delta = jiffies - READ_ONCE(rsp->jiffies_stall);
if (delta < RCU_STALL_RAT_DELAY || !rcu_gp_in_progress(rsp)) {
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
return;
}
WRITE_ONCE(rsp->jiffies_stall,
jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
/* /*
* OK, time to rat on our buddy... * OK, time to rat on our buddy...
* See Documentation/RCU/stallwarn.txt for info on how to debug * See Documentation/RCU/stallwarn.txt for info on how to debug
@ -1441,6 +1428,10 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
sched_show_task(current); sched_show_task(current);
} }
} }
/* Rewrite if needed in case of slow consoles. */
if (ULONG_CMP_GE(jiffies, READ_ONCE(rsp->jiffies_stall)))
WRITE_ONCE(rsp->jiffies_stall,
jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
rcu_check_gp_kthread_starvation(rsp); rcu_check_gp_kthread_starvation(rsp);
@ -1485,6 +1476,7 @@ static void print_cpu_stall(struct rcu_state *rsp)
rcu_dump_cpu_stacks(rsp); rcu_dump_cpu_stacks(rsp);
raw_spin_lock_irqsave_rcu_node(rnp, flags); raw_spin_lock_irqsave_rcu_node(rnp, flags);
/* Rewrite if needed in case of slow consoles. */
if (ULONG_CMP_GE(jiffies, READ_ONCE(rsp->jiffies_stall))) if (ULONG_CMP_GE(jiffies, READ_ONCE(rsp->jiffies_stall)))
WRITE_ONCE(rsp->jiffies_stall, WRITE_ONCE(rsp->jiffies_stall,
jiffies + 3 * rcu_jiffies_till_stall_check() + 3); jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
@ -1508,6 +1500,7 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
unsigned long gpnum; unsigned long gpnum;
unsigned long gps; unsigned long gps;
unsigned long j; unsigned long j;
unsigned long jn;
unsigned long js; unsigned long js;
struct rcu_node *rnp; struct rcu_node *rnp;
@ -1546,14 +1539,17 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
ULONG_CMP_GE(gps, js)) ULONG_CMP_GE(gps, js))
return; /* No stall or GP completed since entering function. */ return; /* No stall or GP completed since entering function. */
rnp = rdp->mynode; rnp = rdp->mynode;
jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
if (rcu_gp_in_progress(rsp) && if (rcu_gp_in_progress(rsp) &&
(READ_ONCE(rnp->qsmask) & rdp->grpmask)) { (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
cmpxchg(&rsp->jiffies_stall, js, jn) == js) {
/* We haven't checked in, so go dump stack. */ /* We haven't checked in, so go dump stack. */
print_cpu_stall(rsp); print_cpu_stall(rsp);
} else if (rcu_gp_in_progress(rsp) && } else if (rcu_gp_in_progress(rsp) &&
ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) { ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
cmpxchg(&rsp->jiffies_stall, js, jn) == js) {
/* They had a few time units to dump stack, so complain. */ /* They had a few time units to dump stack, so complain. */
print_other_cpu_stall(rsp, gpnum); print_other_cpu_stall(rsp, gpnum);