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qemu-timer: check active_timers outside lock/event

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This avoids taking the active_timers_lock or resetting/setting the
timers_done_ev if there are no active timers.  This removes a small
(2-3%) source of overhead for dataplane.  The list is then checked
again inside the lock, or a NULL pointer could be dereferenced.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2016-12-01 09:58:02 +01:00
parent e7a9f35321
commit 8caa05d889
1 changed files with 16 additions and 4 deletions
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20
qemu-timer.c
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@ -174,7 +174,7 @@ void qemu_clock_enable(QEMUClockType type, bool enabled)
bool timerlist_has_timers(QEMUTimerList *timer_list)
{
return !!timer_list->active_timers;
return !!atomic_read(&timer_list->active_timers);
}
bool qemu_clock_has_timers(QEMUClockType type)
@ -187,6 +187,10 @@ bool timerlist_expired(QEMUTimerList *timer_list)
{
int64_t expire_time;
if (!atomic_read(&timer_list->active_timers)) {
return false;
}
qemu_mutex_lock(&timer_list->active_timers_lock);
if (!timer_list->active_timers) {
qemu_mutex_unlock(&timer_list->active_timers_lock);
@ -214,6 +218,10 @@ int64_t timerlist_deadline_ns(QEMUTimerList *timer_list)
int64_t delta;
int64_t expire_time;
if (!atomic_read(&timer_list->active_timers)) {
return -1;
}
if (!timer_list->clock->enabled) {
return -1;
}
@ -363,7 +371,7 @@ static void timer_del_locked(QEMUTimerList *timer_list, QEMUTimer *ts)
if (!t)
break;
if (t == ts) {
*pt = t->next;
atomic_set(pt, t->next);
break;
}
pt = &t->next;
@ -386,7 +394,7 @@ static bool timer_mod_ns_locked(QEMUTimerList *timer_list,
}
ts->expire_time = MAX(expire_time, 0);
ts->next = *pt;
*pt = ts;
atomic_set(pt, ts);
return pt == &timer_list->active_timers;
}
@ -481,8 +489,12 @@ bool timerlist_run_timers(QEMUTimerList *timer_list)
QEMUTimerCB *cb;
void *opaque;
if (!atomic_read(&timer_list->active_timers)) {
return false;
}
qemu_event_reset(&timer_list->timers_done_ev);
if (!timer_list->clock->enabled || !timer_list->active_timers) {
if (!timer_list->clock->enabled) {
goto out;
}