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Message-ID: <CAKfTPtDQOjY9mnVELijktZtN5wLR66yU7gByc2G+-kMg3FsGAg@mail.gmail.com>
Date: Thu, 9 Mar 2023 11:48:07 +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 Thu, 9 Mar 2023 at 09:37, Zhang Qiao <zhangqiao22@...wei.com> wrote:
>
>
>
> 在 2023/3/8 20:55, Vincent Guittot 写道:
> > 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@...wei.com> wrote:
> >>>
> >>>
> >>>
> >>> 在 2023/3/7 18:26, Vincent Guittot 写道:
> >>>> 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
> >>>
> >>> 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)
>
> How about use `se->avg.last_update_time == 0` here?
>
> > + 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)
>
> I tested with this patch and found it make hackbench slower.
Compared to which version ?
- v6.2 + revert commit 829c1651e9c4 ?
- v6.2 ?
v6.2 has a bug because newly migrated task gets a runtime credit
whatever its previous vruntime and hackbench generates a lot of
migration
>
>
> > + 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
> >>>>>>
> >>>> .
> >>>>
> > .
> >
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