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Date: Thu, 2 Mar 2023 15:55:54 +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 15:29, Zhang Qiao <zhangqiao22@...wei.com> wrote:
>
>
>
> 在 2023/3/2 21:34, Vincent Guittot 写道:
> > 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
>
> Hi,
>
> I don't understand when the another migration happend. Could you tell me in more detail?
se->exec_start is update when the task becomes current.
You can have the sequence:
task TA runs on CPU0
TA's se->exec_start = xxxx
TA is put back into the rb tree waiting for next slice while another
task is running
CPU1 pulls TA which migrates on CPU1
migrate_task_rq_fair() w/ TA's se->exec_start == xxxx
TA's se->exec_start = 0
TA is put into the rb tree of CPU1 waiting to run on CPU1
CPU2 pulls TA which migrates on CPU2
migrate_task_rq_fair() w/ TA's se->exec_start == 0
TA's se->exec_start = 0
>
> I think the next migration will happend after the wakee task enqueued, but at this time
> the p->__state isn't TASK_WAKING, p->__state already be changed to TASK_RUNNING at ttwu_do_wakeup().
>
> If such a migration exists, Previous code "se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);" maybe
> perform multiple times，wouldn't it go wrong in this way?
the vruntime have been updated when enqueued but not exec_start
>
> > migrate another time before being scheduled. You should create a
> > helper function like below and use it in both place
>
> Ok, I will update at next version.
>
>
> Thanks,
> ZhangQiao.
>
> >
> > 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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