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Date: Thu, 2 Mar 2023 14:34:19 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Zhang Qiao <zhangqiao22@...wei.com>
Cc: Roman Kagan <rkagan@...zon.de>,
Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org,
Valentin Schneider <vschneid@...hat.com>,
Ben Segall <bsegall@...gle.com>,
Waiman Long <longman@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Mel Gorman <mgorman@...e.de>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed
On Thu, 2 Mar 2023 at 10:36, Zhang Qiao <zhangqiao22@...wei.com> wrote:
>
>
>
> 在 2023/2/27 22:37, Vincent Guittot 写道:
> > On Mon, 27 Feb 2023 at 09:43, Roman Kagan <rkagan@...zon.de> wrote:
> >>
> >> On Tue, Feb 21, 2023 at 06:26:11PM +0100, Vincent Guittot wrote:
> >>> On Tue, 21 Feb 2023 at 17:57, Roman Kagan <rkagan@...zon.de> wrote:
> >>>> What scares me, though, is that I've got a message from the test robot
> >>>> that this commit drammatically affected hackbench results, see the quote
> >>>> below. I expected the commit not to affect any benchmarks.
> >>>>
> >>>> Any idea what could have caused this change?
> >>>
> >>> Hmm, It's most probably because se->exec_start is reset after a
> >>> migration and the condition becomes true for newly migrated task
> >>> whereas its vruntime should be after min_vruntime.
> >>>
> >>> We have missed this condition
> >>
> >> Makes sense to me.
> >>
> >> But what would then be the reliable way to detect a sched_entity which
> >> has slept for long and risks overflowing in .vruntime comparison?
> >
> > For now I don't have a better idea than adding the same check in
> > migrate_task_rq_fair()
>
> Hi, Vincent，
> I fixed this condition as you said, and the test results are as follows.
>
> testcase: hackbench -g 44 -f 20 --process --pipe -l 60000 -s 100
> version1: v6.2
> version2: v6.2 + commit 829c1651e9c4
> version3: v6.2 + commit 829c1651e9c4 + this patch
>
> -------------------------------------------------
> version1 version2 version3
> test1 81.0 118.1 82.1
> test2 82.1 116.9 80.3
> test3 83.2 103.9 83.3
> avg(s) 82.1 113.0 81.9
>
> -------------------------------------------------
> After deal with the task migration case, the hackbench result has restored.
>
> The patch as follow, how does this look?
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index ff4dbbae3b10..3a88d20fd29e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4648,6 +4648,26 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
> #endif
> }
>
> +static inline u64 sched_sleeper_credit(struct sched_entity *se)
> +{
> +
> + unsigned long thresh;
> +
> + if (se_is_idle(se))
> + thresh = sysctl_sched_min_granularity;
> + else
> + thresh = sysctl_sched_latency;
> +
> + /*
> + * Halve their sleep time's effect, to allow
> + * for a gentler effect of sleepers:
> + */
> + if (sched_feat(GENTLE_FAIR_SLEEPERS))
> + thresh >>= 1;
> +
> + return thresh;
> +}
> +
> static void
> place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
> {
> @@ -4664,23 +4684,8 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
> vruntime += sched_vslice(cfs_rq, se);
>
> /* sleeps up to a single latency don't count. */
> - if (!initial) {
> - unsigned long thresh;
> -
> - if (se_is_idle(se))
> - thresh = sysctl_sched_min_granularity;
> - else
> - thresh = sysctl_sched_latency;
> -
> - /*
> - * Halve their sleep time's effect, to allow
> - * for a gentler effect of sleepers:
> - */
> - if (sched_feat(GENTLE_FAIR_SLEEPERS))
> - thresh >>= 1;
> -
> - vruntime -= thresh;
> - }
> + if (!initial)
> + vruntime -= sched_sleeper_credit(se);
>
> /*
> * Pull vruntime of the entity being placed to the base level of
> @@ -4690,7 +4695,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
> * inversed due to s64 overflow.
> */
> sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
> - if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
> + if (se->exec_start != 0 && (s64)sleep_time > 60LL * NSEC_PER_SEC)
> se->vruntime = vruntime;
> else
> se->vruntime = max_vruntime(se->vruntime, vruntime);
> @@ -7634,8 +7639,12 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> */
> if (READ_ONCE(p->__state) == TASK_WAKING) {
> struct cfs_rq *cfs_rq = cfs_rq_of(se);
> + u64 sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
>
> - se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> + if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
You also need to test (se->exec_start !=0) here because the task might
migrate another time before being scheduled. You should create a
helper function like below and use it in both place
static inline bool entity_long_sleep(se)
{
struct cfs_rq *cfs_rq;
u64 sleep_time;
if (se->exec_start == 0)
return false;
cfs_rq = cfs_rq_of(se);
sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
return true;
return false;
}
> + se->vruntime = -sched_sleeper_credit(se);
> + else
> + se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> }
>
> if (!task_on_rq_migrating(p)) {
>
>
>
> Thanks.
> Zhang Qiao.
>
> >
> >>
> >> Thanks,
> >> Roman.
> >>
> >>
> >>
> >> Amazon Development Center Germany GmbH
> >> Krausenstr. 38
> >> 10117 Berlin
> >> Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
> >> Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
> >> Sitz: Berlin
> >> Ust-ID: DE 289 237 879
> >>
> >>
> >>
> > .
> >
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