Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed

[Vincent Guittot]

On Thu, 2 Mar 2023 at 15:29, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>
>
>
> 在 2023/3/2 21:34, Vincent Guittot 写道:
> > On Thu, 2 Mar 2023 at 10:36, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> 在 2023/2/27 22:37, Vincent Guittot 写道:
> >>> On Mon, 27 Feb 2023 at 09:43, Roman Kagan <rkagan@xxxxxxxxx> 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@xxxxxxxxx> 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.
> >>>>
> >>>>
> >>>>
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> >>>>
> >>>>
> >>>>
> >>> .
> >>>
> > .
> >