Date:   Wed, 8 Mar 2023 13:55:33 +0100
From:   Vincent Guittot <vincent.guittot@linaro.org>
To:     Zhang Qiao <zhangqiao22@huawei.com>
Cc:     linux-kernel@vger.kernel.org, mingo@redhat.com,
        peterz@infradead.org, juri.lelli@redhat.com,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        rkagan@amazon.de
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated
Message-ID: <ZAiFxWLSb9HDazSI@vingu-book>
References: <20230306132418.50389-1-zhangqiao22@huawei.com>
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Le mercredi 08 mars 2023 à 09:01:05 (+0100), Vincent Guittot a écrit :
> On Tue, 7 Mar 2023 at 14:41, Zhang Qiao <zhangqiao22@huawei.com> wrote:
> >
> >
> >
> > 在 2023/3/7 18:26, Vincent Guittot 写道:
> > > On Mon, 6 Mar 2023 at 14:53, Vincent Guittot <vincent.guittot@linaro.org> wrote:
> > >>
> > >> On Mon, 6 Mar 2023 at 13:57, Zhang Qiao <zhangqiao22@huawei.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@linaro.org>
> > >>> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
> > >>
> > >> Reviewed-by: Vincent Guittot <vincent.guittot@linaro.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
> >
> > Hi,Vincent,
> >
> > How about using exec_start of the parent sched_entity instant of rq_clock_task()?
> 
> How do we handle sched_entity without a parent sched_entity ?

The change below should do the stuff. Not that we can't use it in place entity because
pelt is not enabled in UP.

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 74c9918ffe76..b8b381b0ff20 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7635,6 +7635,32 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
        return new_cpu;
 }

+static inline bool migrate_long_sleeper(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);
+       /*
+        * If the entity slept for a long time, don't even try to normalize its
+        * vruntime with the base as it may be too far off and might generate
+        * wrong decision because of s64 overflow.
+        * We estimate its sleep duration with the last update of se's pelt.
+        * The last update happened before sleeping. The cfs' pelt is not
+        * always updated when cfs is idle but this is not a problem because
+        * its min_vruntime is not updated too, so the situation can't get
+        * worse.
+        */
+       sleep_time = cfs_rq_last_update_time(cfs_rq) - se->avg.last_update_time;
+       if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
+               return true;
+
+       return false;
+}
+
 /*
  * Called immediately before a task is migrated to a new CPU; task_cpu(p) and
  * cfs_rq_of(p) references at time of call are still valid and identify the
@@ -7666,7 +7692,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
                 * enqueue, we only need to reset the se->vruntime of waking task
                 * to a credit here.
                 */
-               if (entity_is_long_sleep(se))
+               if (migrate_long_sleeper(se))
                        se->vruntime = -sched_sleeper_credit(se);
                else
                        se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);


> 
> 
> 
> > >
> > >>> +                       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
> > >>>
> > > .
> > >