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sched: Strengthen buddies and mitigate buddy induced latencies

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This patch restores the effectiveness of LAST_BUDDY in preventing
pgsql+oltp from collapsing due to wakeup preemption. It also
switches LAST_BUDDY to exclusively do what it does best, namely
mitigate the effects of aggressive wakeup preemption, which
improves vmark throughput markedly, and restores mysql+oltp
scalability.

Since buddies are about scalability, enable them beginning at the
point where we begin expanding sched_latency, namely
sched_nr_latency. Previously, buddies were cleared aggressively,
which seriously reduced their effectiveness. Not clearing
aggressively however, produces a small drop in mysql+oltp
throughput immediately after peak, indicating that LAST_BUDDY is
actually doing some harm. This is right at the point where X on the
desktop in competition with another load wants low latency service.
Ergo, do not enable until we need to scale.

To mitigate latency induced by buddies, or by a task just missing
wakeup preemption, check latency at tick time.

Last hunk prevents buddies from stymieing BALANCE_NEWIDLE via
CACHE_HOT_BUDDY.

Supporting performance tests:

 tip   = v2.6.32-rc5-1497-ga525b32
 tipx  = NO_GENTLE_FAIR_SLEEPERS NEXT_BUDDY granularity knobs = 31 knobs + 31 buddies
 tip+x = NO_GENTLE_FAIR_SLEEPERS granularity knobs = 31 knobs

(Three run averages except where noted.)

 vmark:
 ------
 tip           108466 messages per second
 tip+          125307 messages per second
 tip+x         125335 messages per second
 tipx          117781 messages per second
 2.6.31.3      122729 messages per second

 mysql+oltp:
 -----------
 clients          1        2        4        8       16       32       64        128    256
 ..........................................................................................
 tip        9949.89 18690.20 34801.24 34460.04 32682.88 30765.97 28305.27 25059.64 19548.08
 tip+      10013.90 18526.84 34900.38 34420.14 33069.83 32083.40 30578.30 28010.71 25605.47
 tipx       9698.71 18002.70 34477.56 33420.01 32634.30 31657.27 29932.67 26827.52 21487.18
 2.6.31.3   8243.11 18784.20 34404.83 33148.38 31900.32 31161.90 29663.81 25995.94 18058.86

 pgsql+oltp:
 -----------
 clients          1        2        4        8       16       32       64      128      256
 ..........................................................................................
 tip       13686.37 26609.25 51934.28 51347.81 49479.51 45312.65 36691.91 26851.57 24145.35
 tip+ (1x) 13907.85 27135.87 52951.98 52514.04 51742.52 50705.43 49947.97 48374.19 46227.94
 tip+x     13906.78 27065.81 52951.19 52542.59 52176.11 51815.94 50838.90 49439.46 46891.00
 tipx      13742.46 26769.81 52351.99 51891.73 51320.79 50938.98 50248.65 48908.70 46553.84
 2.6.31.3  13815.35 26906.46 52683.34 52061.31 51937.10 51376.80 50474.28 49394.47 47003.25

Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Mike Galbraith 2009-10-23 23:09:22 +02:00 committed by Ingo Molnar
parent 92f6a5e37a
commit f685ceacab
2 changed files with 48 additions and 27 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
kernel/sched.c
View File

@ -2008,7 +2008,7 @@ task_hot(struct task_struct *p, u64 now, struct sched_domain *sd)
/*
* Buddy candidates are cache hot:
*/
if (sched_feat(CACHE_HOT_BUDDY) &&
if (sched_feat(CACHE_HOT_BUDDY) && this_rq()->nr_running &&
(&p->se == cfs_rq_of(&p->se)->next ||
&p->se == cfs_rq_of(&p->se)->last))
return 1;

73
kernel/sched_fair.c
View File

@ -822,6 +822,26 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
* re-elected due to buddy favours.
*/
clear_buddies(cfs_rq, curr);
return;
}
/*
* Ensure that a task that missed wakeup preemption by a
* narrow margin doesn't have to wait for a full slice.
* This also mitigates buddy induced latencies under load.
*/
if (!sched_feat(WAKEUP_PREEMPT))
return;
if (delta_exec < sysctl_sched_min_granularity)
return;
if (cfs_rq->nr_running > 1) {
struct sched_entity *se = __pick_next_entity(cfs_rq);
s64 delta = curr->vruntime - se->vruntime;
if (delta > ideal_runtime)
resched_task(rq_of(cfs_rq)->curr);
}
}
@ -861,21 +881,18 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
{
struct sched_entity *se = __pick_next_entity(cfs_rq);
struct sched_entity *buddy;
struct sched_entity *left = se;
if (cfs_rq->next) {
buddy = cfs_rq->next;
cfs_rq->next = NULL;
if (wakeup_preempt_entity(buddy, se) < 1)
return buddy;
}
if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
se = cfs_rq->next;
if (cfs_rq->last) {
buddy = cfs_rq->last;
cfs_rq->last = NULL;
if (wakeup_preempt_entity(buddy, se) < 1)
return buddy;
}
/*
* Prefer last buddy, try to return the CPU to a preempted task.
*/
if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
se = cfs_rq->last;
clear_buddies(cfs_rq, se);
return se;
}
@ -1577,6 +1594,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
struct sched_entity *se = &curr->se, *pse = &p->se;
struct cfs_rq *cfs_rq = task_cfs_rq(curr);
int sync = wake_flags & WF_SYNC;
int scale = cfs_rq->nr_running >= sched_nr_latency;
update_curr(cfs_rq);
@ -1591,18 +1609,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
if (unlikely(se == pse))
return;
/*
* Only set the backward buddy when the current task is still on the
* rq. This can happen when a wakeup gets interleaved with schedule on
* the ->pre_schedule() or idle_balance() point, either of which can
* drop the rq lock.
*
* Also, during early boot the idle thread is in the fair class, for
* obvious reasons its a bad idea to schedule back to the idle thread.
*/
if (sched_feat(LAST_BUDDY) && likely(se->on_rq && curr != rq->idle))
set_last_buddy(se);
if (sched_feat(NEXT_BUDDY) && !(wake_flags & WF_FORK))
if (sched_feat(NEXT_BUDDY) && scale && !(wake_flags & WF_FORK))
set_next_buddy(pse);
/*
@ -1648,8 +1655,22 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
BUG_ON(!pse);
if (wakeup_preempt_entity(se, pse) == 1)
if (wakeup_preempt_entity(se, pse) == 1) {
resched_task(curr);
/*
* Only set the backward buddy when the current task is still
* on the rq. This can happen when a wakeup gets interleaved
* with schedule on the ->pre_schedule() or idle_balance()
* point, either of which can * drop the rq lock.
*
* Also, during early boot the idle thread is in the fair class,
* for obvious reasons its a bad idea to schedule back to it.
*/
if (unlikely(!se->on_rq || curr == rq->idle))
return;
if (sched_feat(LAST_BUDDY) && scale && entity_is_task(se))
set_last_buddy(se);
}
}
static struct task_struct *pick_next_task_fair(struct rq *rq)