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Date: Wed, 8 Mar 2023 13:55:33 +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
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)
+ 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
> > >>>
> > > .
> > >
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