Date:   Tue, 7 Mar 2023 11:26:18 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Zhang Qiao <zhangqiao22@...wei.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        rkagan@...zon.de
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

On Mon, 6 Mar 2023 at 14:53, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>
> On Mon, 6 Mar 2023 at 13:57, Zhang Qiao <zhangqiao22@...wei.com> wrote:
> >
> > Commit 829c1651e9c4 ("sched/fair: sanitize vruntime of
> > entity being placed") fix an overflowing bug, but ignore
> > a case that se->exec_start is reset after a migration.
> >
> > For fixing this case, we reset the vruntime of a long
> > sleeping task in migrate_task_rq_fair().
> >
> > Fixes: 829c1651e9c4 ("sched/fair: sanitize vruntime of entity being placed")
> > Suggested-by: Vincent Guittot <vincent.guittot@...aro.org>
> > Signed-off-by: Zhang Qiao <zhangqiao22@...wei.com>
>
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
>
> > ---
> >
> > v1 -> v2:
> > - fix some typos and update comments
> > - reformat the patch
> >
> > ---
> >  kernel/sched/fair.c | 76 ++++++++++++++++++++++++++++++++-------------
> >  1 file changed, 55 insertions(+), 21 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 7a1b1f855b96..74c9918ffe76 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4648,11 +4648,45 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >  #endif
> >  }
> >
> > +static inline bool entity_is_long_sleep(struct sched_entity *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;
> > +}
> > +
> > +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)
> >  {
> >         u64 vruntime = cfs_rq->min_vruntime;
> > -       u64 sleep_time;
> >
> >         /*
> >          * The 'current' period is already promised to the current tasks,
> > @@ -4664,23 +4698,8 @@ 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
> > @@ -4689,8 +4708,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
> >          * the base as it may be too far off and the comparison may get
> >          * 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 (entity_is_long_sleep(se))
> >                 se->vruntime = vruntime;
> >         else
> >                 se->vruntime = max_vruntime(se->vruntime, vruntime);
> > @@ -7635,7 +7653,23 @@ 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);
> >
> > -               se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> > +               /*
> > +                * We determine whether a task sleeps for long by checking
> > +                * se->exec_start, and if it is, we sanitize its vruntime at
> > +                * place_entity(). However, after a migration, this detection
> > +                * method fails due to se->exec_start being reset.
> > +                *
> > +                * For fixing this case, we add the same check here. For a task
> > +                * which has slept for a long time, its vruntime should be reset
> > +                * to cfs_rq->min_vruntime with a sleep credit. Because waking
> > +                * task's vruntime will be added to cfs_rq->min_vruntime when
> > +                * enqueue, we only need to reset the se->vruntime of waking task
> > +                * to a credit here.
> > +                */
> > +               if (entity_is_long_sleep(se))

I completely overlooked that we can't use rq_clock_task here. Need to
think a bit more on this

> > +                       se->vruntime = -sched_sleeper_credit(se);
> > +               else
> > +                       se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> >         }
> >
> >         if (!task_on_rq_migrating(p)) {
> > --
> > 2.17.1
> >

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