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cpus-common: lock-free fast path for cpu_exec_start/end 

Set cpu->running without taking the cpu_list lock, only requiring it if
there is a concurrent exclusive section.  This requires adding a new
field to CPUState, which records whether a running CPU is being counted
in pending_cpus.

When an exclusive section is started concurrently with cpu_exec_start,
cpu_exec_start can use the new field to determine if it has to wait for
the end of the exclusive section.  Likewise, cpu_exec_end can use it to
see if start_exclusive is waiting for that CPU.

This a separate patch for easier bisection of issues.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2016-08-31 21:33:58 +02:00
parent 3359baad36
commit c265e976f4
3 changed files with 133 additions and 20 deletions
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95
cpus-common.c
View File

@ -28,6 +28,9 @@ static QemuCond exclusive_cond;
static QemuCond exclusive_resume;
static QemuCond qemu_work_cond;
/* >= 1 if a thread is inside start_exclusive/end_exclusive. Written
* under qemu_cpu_list_lock, read with atomic operations.
*/
static int pending_cpus;
void qemu_init_cpu_list(void)
@ -177,18 +180,26 @@ static inline void exclusive_idle(void)
void start_exclusive(void)
{
CPUState *other_cpu;
int running_cpus;
qemu_mutex_lock(&qemu_cpu_list_lock);
exclusive_idle();
/* Make all other cpus stop executing. */
pending_cpus = 1;
atomic_set(&pending_cpus, 1);
/* Write pending_cpus before reading other_cpu->running. */
smp_mb();
running_cpus = 0;
CPU_FOREACH(other_cpu) {
if (other_cpu->running) {
pending_cpus++;
if (atomic_read(&other_cpu->running)) {
other_cpu->has_waiter = true;
running_cpus++;
qemu_cpu_kick(other_cpu);
}
}
atomic_set(&pending_cpus, running_cpus + 1);
while (pending_cpus > 1) {
qemu_cond_wait(&exclusive_cond, &qemu_cpu_list_lock);
}
@ -203,7 +214,7 @@ void start_exclusive(void)
void end_exclusive(void)
{
qemu_mutex_lock(&qemu_cpu_list_lock);
pending_cpus = 0;
atomic_set(&pending_cpus, 0);
qemu_cond_broadcast(&exclusive_resume);
qemu_mutex_unlock(&qemu_cpu_list_lock);
}
@ -211,24 +222,78 @@ void end_exclusive(void)
/* Wait for exclusive ops to finish, and begin cpu execution. */
void cpu_exec_start(CPUState *cpu)
{
qemu_mutex_lock(&qemu_cpu_list_lock);
exclusive_idle();
cpu->running = true;
qemu_mutex_unlock(&qemu_cpu_list_lock);
atomic_set(&cpu->running, true);
/* Write cpu->running before reading pending_cpus. */
smp_mb();
/* 1. start_exclusive saw cpu->running == true and pending_cpus >= 1.
* After taking the lock we'll see cpu->has_waiter == true and run---not
* for long because start_exclusive kicked us. cpu_exec_end will
* decrement pending_cpus and signal the waiter.
*
* 2. start_exclusive saw cpu->running == false but pending_cpus >= 1.
* This includes the case when an exclusive item is running now.
* Then we'll see cpu->has_waiter == false and wait for the item to
* complete.
*
* 3. pending_cpus == 0. Then start_exclusive is definitely going to
* see cpu->running == true, and it will kick the CPU.
*/
if (unlikely(atomic_read(&pending_cpus))) {
qemu_mutex_lock(&qemu_cpu_list_lock);
if (!cpu->has_waiter) {
/* Not counted in pending_cpus, let the exclusive item
* run. Since we have the lock, just set cpu->running to true
* while holding it; no need to check pending_cpus again.
*/
atomic_set(&cpu->running, false);
exclusive_idle();
/* Now pending_cpus is zero. */
atomic_set(&cpu->running, true);
} else {
/* Counted in pending_cpus, go ahead and release the
* waiter at cpu_exec_end.
*/
}
qemu_mutex_unlock(&qemu_cpu_list_lock);
}
}
/* Mark cpu as not executing, and release pending exclusive ops. */
void cpu_exec_end(CPUState *cpu)
{
qemu_mutex_lock(&qemu_cpu_list_lock);
cpu->running = false;
if (pending_cpus > 1) {
pending_cpus--;
if (pending_cpus == 1) {
qemu_cond_signal(&exclusive_cond);
atomic_set(&cpu->running, false);
/* Write cpu->running before reading pending_cpus. */
smp_mb();
/* 1. start_exclusive saw cpu->running == true. Then it will increment
* pending_cpus and wait for exclusive_cond. After taking the lock
* we'll see cpu->has_waiter == true.
*
* 2. start_exclusive saw cpu->running == false but here pending_cpus >= 1.
* This includes the case when an exclusive item started after setting
* cpu->running to false and before we read pending_cpus. Then we'll see
* cpu->has_waiter == false and not touch pending_cpus. The next call to
* cpu_exec_start will run exclusive_idle if still necessary, thus waiting
* for the item to complete.
*
* 3. pending_cpus == 0. Then start_exclusive is definitely going to
* see cpu->running == false, and it can ignore this CPU until the
* next cpu_exec_start.
*/
if (unlikely(atomic_read(&pending_cpus))) {
qemu_mutex_lock(&qemu_cpu_list_lock);
if (cpu->has_waiter) {
cpu->has_waiter = false;
atomic_set(&pending_cpus, pending_cpus - 1);
if (pending_cpus == 1) {
qemu_cond_signal(&exclusive_cond);
}
}
qemu_mutex_unlock(&qemu_cpu_list_lock);
}
qemu_mutex_unlock(&qemu_cpu_list_lock);
}
void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data)

53
docs/tcg-exclusive.promela
View File

@ -13,7 +13,8 @@
* gcc pan.c -O2
* ./a.out -a
*
* Tunable processor macros: N_CPUS, N_EXCLUSIVE, N_CYCLES, TEST_EXPENSIVE.
* Tunable processor macros: N_CPUS, N_EXCLUSIVE, N_CYCLES, USE_MUTEX,
* TEST_EXPENSIVE.
*/
// Define the missing parameters for the model
@ -22,8 +23,10 @@
#warning defaulting to 2 CPU processes
#endif
// the expensive test is not so expensive for <= 3 CPUs
#if N_CPUS <= 3
// the expensive test is not so expensive for <= 2 CPUs
// If the mutex is used, it's also cheap (300 MB / 4 seconds) for 3 CPUs
// For 3 CPUs and the lock-free option it needs 1.5 GB of RAM
#if N_CPUS <= 2 || (N_CPUS <= 3 && defined USE_MUTEX)
#define TEST_EXPENSIVE
#endif
@ -107,6 +110,8 @@ byte has_waiter[N_CPUS];
COND_BROADCAST(exclusive_resume); \
MUTEX_UNLOCK(mutex);
#ifdef USE_MUTEX
// Simple version using mutexes
#define cpu_exec_start(id) \
MUTEX_LOCK(mutex); \
exclusive_idle(); \
@ -127,6 +132,48 @@ byte has_waiter[N_CPUS];
:: else -> skip; \
fi; \
MUTEX_UNLOCK(mutex);
#else
// Wait-free fast path, only needs mutex when concurrent with
// an exclusive section
#define cpu_exec_start(id) \
running[id] = 1; \
if \
:: pending_cpus -> { \
MUTEX_LOCK(mutex); \
if \
:: !has_waiter[id] -> { \
running[id] = 0; \
exclusive_idle(); \
running[id] = 1; \
} \
:: else -> skip; \
fi; \
MUTEX_UNLOCK(mutex); \
} \
:: else -> skip; \
fi;
#define cpu_exec_end(id) \
running[id] = 0; \
if \
:: pending_cpus -> { \
MUTEX_LOCK(mutex); \
if \
:: has_waiter[id] -> { \
has_waiter[id] = 0; \
pending_cpus--; \
if \
:: pending_cpus == 1 -> COND_BROADCAST(exclusive_cond); \
:: else -> skip; \
fi; \
} \
:: else -> skip; \
fi; \
MUTEX_UNLOCK(mutex); \
} \
:: else -> skip; \
fi
#endif
// Promela processes

5
include/qom/cpu.h
View File

@ -242,7 +242,8 @@ struct qemu_work_item;
* @nr_threads: Number of threads within this CPU.
* @numa_node: NUMA node this CPU is belonging to.
* @host_tid: Host thread ID.
* @running: #true if CPU is currently running;
* @running: #true if CPU is currently running (lockless).
* @has_waiter: #true if a CPU is currently waiting for the cpu_exec_end;
* valid under cpu_list_lock.
* @created: Indicates whether the CPU thread has been successfully created.
* @interrupt_request: Indicates a pending interrupt request.
@ -296,7 +297,7 @@ struct CPUState {
#endif
int thread_id;
uint32_t host_tid;
bool running;
bool running, has_waiter;
struct QemuCond *halt_cond;
bool thread_kicked;
bool created;