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[PATCH] sched: fix smt nice lock contention and optimization

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Initial report and lock contention fix from Chris Mason:

Recent benchmarks showed some performance regressions between 2.6.16 and
2.6.5.  We tracked down one of the regressions to lock contention in
schedule heavy workloads (~70,000 context switches per second)

kernel/sched.c:dependent_sleeper() was responsible for most of the lock
contention, hammering on the run queue locks.  The patch below is more of a
discussion point than a suggested fix (although it does reduce lock
contention significantly).  The dependent_sleeper code looks very expensive
to me, especially for using a spinlock to bounce control between two
different siblings in the same cpu.

It is further optimized:

* perform dependent_sleeper check after next task is determined
* convert wake_sleeping_dependent to use trylock
* skip smt runqueue check if trylock fails
* optimize double_rq_lock now that smt nice is converted to trylock
* early exit in searching first SD_SHARE_CPUPOWER domain
* speedup fast path of dependent_sleeper

[akpm@osdl.org: cleanup]
Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Con Kolivas <kernel@kolivas.org>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Chris Mason <mason@suse.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Chen, Kenneth W 2006-06-27 02:54:28 -07:00 committed by Linus Torvalds
parent 7a8e2a5ea4
commit c96d145e71
1 changed files with 60 additions and 124 deletions
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184
kernel/sched.c
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@ -239,7 +239,6 @@ struct runqueue {
task_t *migration_thread;
struct list_head migration_queue;
int cpu;
#endif
#ifdef CONFIG_SCHEDSTATS
@ -1074,9 +1073,10 @@ static int sched_balance_self(int cpu, int flag)
struct task_struct *t = current;
struct sched_domain *tmp, *sd = NULL;
for_each_domain(cpu, tmp)
for_each_domain(cpu, tmp) {
if (tmp->flags & flag)
sd = tmp;
}
while (sd) {
cpumask_t span;
@ -1691,9 +1691,6 @@ unsigned long nr_active(void)
/*
* double_rq_lock - safely lock two runqueues
*
* We must take them in cpu order to match code in
* dependent_sleeper and wake_dependent_sleeper.
*
* Note this does not disable interrupts like task_rq_lock,
* you need to do so manually before calling.
*/
@ -1705,7 +1702,7 @@ static void double_rq_lock(runqueue_t *rq1, runqueue_t *rq2)
spin_lock(&rq1->lock);
__acquire(rq2->lock); /* Fake it out ;) */
} else {
if (rq1->cpu < rq2->cpu) {
if (rq1 < rq2) {
spin_lock(&rq1->lock);
spin_lock(&rq2->lock);
} else {
@ -1741,7 +1738,7 @@ static void double_lock_balance(runqueue_t *this_rq, runqueue_t *busiest)
__acquires(this_rq->lock)
{
if (unlikely(!spin_trylock(&busiest->lock))) {
if (busiest->cpu < this_rq->cpu) {
if (busiest < this_rq) {
spin_unlock(&this_rq->lock);
spin_lock(&busiest->lock);
spin_lock(&this_rq->lock);
@ -2352,10 +2349,11 @@ static void active_load_balance(runqueue_t *busiest_rq, int busiest_cpu)
double_lock_balance(busiest_rq, target_rq);
/* Search for an sd spanning us and the target CPU. */
for_each_domain(target_cpu, sd)
for_each_domain(target_cpu, sd) {
if ((sd->flags & SD_LOAD_BALANCE) &&
cpu_isset(busiest_cpu, sd->span))
break;
}
if (unlikely(sd == NULL))
goto out;
@ -2691,48 +2689,35 @@ static inline void wakeup_busy_runqueue(runqueue_t *rq)
resched_task(rq->idle);
}
static void wake_sleeping_dependent(int this_cpu, runqueue_t *this_rq)
/*
* Called with interrupt disabled and this_rq's runqueue locked.
*/
static void wake_sleeping_dependent(int this_cpu)
{
struct sched_domain *tmp, *sd = NULL;
cpumask_t sibling_map;
int i;
for_each_domain(this_cpu, tmp)
if (tmp->flags & SD_SHARE_CPUPOWER)
for_each_domain(this_cpu, tmp) {
if (tmp->flags & SD_SHARE_CPUPOWER) {
sd = tmp;
break;
}
}
if (!sd)
return;
/*
* Unlock the current runqueue because we have to lock in
* CPU order to avoid deadlocks. Caller knows that we might
* unlock. We keep IRQs disabled.
*/
spin_unlock(&this_rq->lock);
sibling_map = sd->span;
for_each_cpu_mask(i, sibling_map)
spin_lock(&cpu_rq(i)->lock);
/*
* We clear this CPU from the mask. This both simplifies the
* inner loop and keps this_rq locked when we exit:
*/
cpu_clear(this_cpu, sibling_map);
for_each_cpu_mask(i, sibling_map) {
for_each_cpu_mask(i, sd->span) {
runqueue_t *smt_rq = cpu_rq(i);
wakeup_busy_runqueue(smt_rq);
}
if (i == this_cpu)
continue;
if (unlikely(!spin_trylock(&smt_rq->lock)))
continue;
for_each_cpu_mask(i, sibling_map)
spin_unlock(&cpu_rq(i)->lock);
/*
* We exit with this_cpu's rq still held and IRQs
* still disabled:
*/
wakeup_busy_runqueue(smt_rq);
spin_unlock(&smt_rq->lock);
}
}
/*
@ -2745,52 +2730,46 @@ static inline unsigned long smt_slice(task_t *p, struct sched_domain *sd)
return p->time_slice * (100 - sd->per_cpu_gain) / 100;
}
static int dependent_sleeper(int this_cpu, runqueue_t *this_rq)
/*
* To minimise lock contention and not have to drop this_rq's runlock we only
* trylock the sibling runqueues and bypass those runqueues if we fail to
* acquire their lock. As we only trylock the normal locking order does not
* need to be obeyed.
*/
static int dependent_sleeper(int this_cpu, runqueue_t *this_rq, task_t *p)
{
struct sched_domain *tmp, *sd = NULL;
cpumask_t sibling_map;
prio_array_t *array;
int ret = 0, i;
task_t *p;
for_each_domain(this_cpu, tmp)
if (tmp->flags & SD_SHARE_CPUPOWER)
/* kernel/rt threads do not participate in dependent sleeping */
if (!p->mm || rt_task(p))
return 0;
for_each_domain(this_cpu, tmp) {
if (tmp->flags & SD_SHARE_CPUPOWER) {
sd = tmp;
break;
}
}
if (!sd)
return 0;
/*
* The same locking rules and details apply as for
* wake_sleeping_dependent():
*/
spin_unlock(&this_rq->lock);
sibling_map = sd->span;
for_each_cpu_mask(i, sibling_map)
spin_lock(&cpu_rq(i)->lock);
cpu_clear(this_cpu, sibling_map);
for_each_cpu_mask(i, sd->span) {
runqueue_t *smt_rq;
task_t *smt_curr;
/*
* Establish next task to be run - it might have gone away because
* we released the runqueue lock above:
*/
if (!this_rq->nr_running)
goto out_unlock;
array = this_rq->active;
if (!array->nr_active)
array = this_rq->expired;
BUG_ON(!array->nr_active);
if (i == this_cpu)
continue;
p = list_entry(array->queue[sched_find_first_bit(array->bitmap)].next,
task_t, run_list);
smt_rq = cpu_rq(i);
if (unlikely(!spin_trylock(&smt_rq->lock)))
continue;
for_each_cpu_mask(i, sibling_map) {
runqueue_t *smt_rq = cpu_rq(i);
task_t *smt_curr = smt_rq->curr;
smt_curr = smt_rq->curr;
/* Kernel threads do not participate in dependent sleeping */
if (!p->mm || !smt_curr->mm || rt_task(p))
goto check_smt_task;
if (!smt_curr->mm)
goto unlock;
/*
* If a user task with lower static priority than the
@ -2808,49 +2787,24 @@ static int dependent_sleeper(int this_cpu, runqueue_t *this_rq)
if ((jiffies % DEF_TIMESLICE) >
(sd->per_cpu_gain * DEF_TIMESLICE / 100))
ret = 1;
} else
} else {
if (smt_curr->static_prio < p->static_prio &&
!TASK_PREEMPTS_CURR(p, smt_rq) &&
smt_slice(smt_curr, sd) > task_timeslice(p))
ret = 1;
check_smt_task:
if ((!smt_curr->mm && smt_curr != smt_rq->idle) ||
rt_task(smt_curr))
continue;
if (!p->mm) {
wakeup_busy_runqueue(smt_rq);
continue;
}
/*
* Reschedule a lower priority task on the SMT sibling for
* it to be put to sleep, or wake it up if it has been put to
* sleep for priority reasons to see if it should run now.
*/
if (rt_task(p)) {
if ((jiffies % DEF_TIMESLICE) >
(sd->per_cpu_gain * DEF_TIMESLICE / 100))
resched_task(smt_curr);
} else {
if (TASK_PREEMPTS_CURR(p, smt_rq) &&
smt_slice(p, sd) > task_timeslice(smt_curr))
resched_task(smt_curr);
else
wakeup_busy_runqueue(smt_rq);
}
unlock:
spin_unlock(&smt_rq->lock);
}
out_unlock:
for_each_cpu_mask(i, sibling_map)
spin_unlock(&cpu_rq(i)->lock);
return ret;
}
#else
static inline void wake_sleeping_dependent(int this_cpu, runqueue_t *this_rq)
static inline void wake_sleeping_dependent(int this_cpu)
{
}
static inline int dependent_sleeper(int this_cpu, runqueue_t *this_rq)
static inline int dependent_sleeper(int this_cpu, runqueue_t *this_rq,
task_t *p)
{
return 0;
}
@ -2972,32 +2926,13 @@ need_resched_nonpreemptible:
cpu = smp_processor_id();
if (unlikely(!rq->nr_running)) {
go_idle:
idle_balance(cpu, rq);
if (!rq->nr_running) {
next = rq->idle;
rq->expired_timestamp = 0;
wake_sleeping_dependent(cpu, rq);
/*
* wake_sleeping_dependent() might have released
* the runqueue, so break out if we got new
* tasks meanwhile:
*/
if (!rq->nr_running)
goto switch_tasks;
}
} else {
if (dependent_sleeper(cpu, rq)) {
next = rq->idle;
wake_sleeping_dependent(cpu);
goto switch_tasks;
}
/*
* dependent_sleeper() releases and reacquires the runqueue
* lock, hence go into the idle loop if the rq went
* empty meanwhile:
*/
if (unlikely(!rq->nr_running))
goto go_idle;
}
array = rq->active;
@ -3035,6 +2970,8 @@ go_idle:
}
}
next->sleep_type = SLEEP_NORMAL;
if (dependent_sleeper(cpu, rq, next))
next = rq->idle;
switch_tasks:
if (next == rq->idle)
schedstat_inc(rq, sched_goidle);
@ -6144,7 +6081,6 @@ void __init sched_init(void)
rq->push_cpu = 0;
rq->migration_thread = NULL;
INIT_LIST_HEAD(&rq->migration_queue);
rq->cpu = i;
#endif
atomic_set(&rq->nr_iowait, 0);
@ -6205,7 +6141,7 @@ void normalize_rt_tasks(void)
runqueue_t *rq;
read_lock_irq(&tasklist_lock);
for_each_process (p) {
for_each_process(p) {
if (!rt_task(p))
continue;