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Message-ID: <20250523094106.GA1210419@bytedance>
Date: Fri, 23 May 2025 17:42:05 +0800
From: Aaron Lu <ziqianlu@...edance.com>
To: Chengming Zhou <chengming.zhou@...ux.dev>
Cc: Valentin Schneider <vschneid@...hat.com>,
Ben Segall <bsegall@...gle.com>,
K Prateek Nayak <kprateek.nayak@....com>,
Peter Zijlstra <peterz@...radead.org>,
Josh Don <joshdon@...gle.com>, Ingo Molnar <mingo@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Xi Wang <xii@...gle.com>, linux-kernel@...r.kernel.org,
Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>, Mel Gorman <mgorman@...e.de>,
Chuyi Zhou <zhouchuyi@...edance.com>,
Jan Kiszka <jan.kiszka@...mens.com>,
Florian Bezdeka <florian.bezdeka@...mens.com>
Subject: Re: [PATCH 4/7] sched/fair: Take care of group/affinity/sched_class
change for throttled task
On Fri, May 23, 2025 at 05:13:35PM +0800, Chengming Zhou wrote:
> On 2025/5/23 15:56, Aaron Lu wrote:
> > On Fri, May 23, 2025 at 10:43:53AM +0800, Chengming Zhou wrote:
> > > On 2025/5/20 18:41, Aaron Lu wrote:
> > > > On task group change, for tasks whose on_rq equals to TASK_ON_RQ_QUEUED,
> > > > core will dequeue it and then requeued it.
> > > >
> > > > The throttled task is still considered as queued by core because p->on_rq
> > > > is still set so core will dequeue it, but since the task is already
> > > > dequeued on throttle in fair, handle this case properly.
> > > >
> > > > Affinity and sched class change is similar.
> > >
> > > How about setting p->on_rq to 0 when throttled? which is the fact that
> > > the task is not on cfs queue anymore, does this method cause any problem?
> > >
> >
> > On task group change/affinity change etc. if the throttled task is
> > regarded as !on_rq, then it will miss the chance to be enqueued to the
> > new(and correct) cfs_rqs, instead, it will be enqueued back to its
> > original cfs_rq on unthrottle which breaks affinity or task group
>
> Yeah, this is indeed a problem, I was thinking to delete the throttled task
> from the cfs_rq limbo list, then add it to another cfs_rq limbo list or cfs_rq
> runnable tree based on the new cfs_rq's throttle status.
Only work when the task is still handled by fair :)
>
> But it's much complex compared with your current method.
>
> > settings. We may be able to do something in tg_unthrottle_up() to take
> > special care of these situations, but it seems a lot of headaches.
> >
> > Also, for task group change, if the new task group does not have throttle
> > setting, that throttled task should be allowed to run immediately instead
> > of waiting for its old cfs_rq's unthrottle event. Similar is true when
> > this throttled task changed its sched class, like from fair to rt.
> >
> > Makes sense?
>
> Ok, the another problem of the current method I can think of is the PELT maintenance,
> we skip the actual dequeue_task_fair() process, which includes PELT detach, we just
> delete it from the cfs_rq limbo list, so it can result in PELT maintenance error.
>
There are corresponding callbacks that handle this, e.g. for task group
change, there is task_change_group_fair() that handles PELT detach; for
affinity change, there is migrate_task_rq_fair() does that and for sched
class change, there is switched_from/to() does that.
Or do I miss anything?
Thanks,
Aaron
> > > >
> > > > Signed-off-by: Aaron Lu <ziqianlu@...edance.com>
> > > > ---
> > > > kernel/sched/fair.c | 24 ++++++++++++++++++++++++
> > > > 1 file changed, 24 insertions(+)
> > > >
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index 74bc320cbc238..4c66fd8d24389 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -5866,6 +5866,10 @@ static void throttle_cfs_rq_work(struct callback_head *work)
> > > > update_rq_clock(rq);
> > > > WARN_ON_ONCE(!list_empty(&p->throttle_node));
> > > > dequeue_task_fair(rq, p, DEQUEUE_SLEEP | DEQUEUE_SPECIAL);
> > > > + /*
> > > > + * Must not add it to limbo list before dequeue or dequeue will
> > > > + * mistakenly regard this task as an already throttled one.
> > > > + */
> > > > list_add(&p->throttle_node, &cfs_rq->throttled_limbo_list);
> > > > resched_curr(rq);
> > > > }
> > > > @@ -5881,6 +5885,20 @@ void init_cfs_throttle_work(struct task_struct *p)
> > > > INIT_LIST_HEAD(&p->throttle_node);
> > > > }
> > > > +static void dequeue_throttled_task(struct task_struct *p, int flags)
> > > > +{
> > > > + /*
> > > > + * Task is throttled and someone wants to dequeue it again:
> > > > + * it must be sched/core when core needs to do things like
> > > > + * task affinity change, task group change, task sched class
> > > > + * change etc.
> > > > + */
> > > > + WARN_ON_ONCE(p->se.on_rq);
> > > > + WARN_ON_ONCE(flags & DEQUEUE_SLEEP);
> > > > +
> > > > + list_del_init(&p->throttle_node);
> > > > +}
> > > > +
> > > > static void enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags);
> > > > static int tg_unthrottle_up(struct task_group *tg, void *data)
> > > > {
> > > > @@ -6834,6 +6852,7 @@ static inline void sync_throttle(struct task_group *tg, int cpu) {}
> > > > static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq) {}
> > > > static void task_throttle_setup_work(struct task_struct *p) {}
> > > > static bool task_is_throttled(struct task_struct *p) { return false; }
> > > > +static void dequeue_throttled_task(struct task_struct *p, int flags) {}
> > > > static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
> > > > {
> > > > @@ -7281,6 +7300,11 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
> > > > */
> > > > static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> > > > {
> > > > + if (unlikely(task_is_throttled(p))) {
> > > > + dequeue_throttled_task(p, flags);
> > > > + return true;
> > > > + }
> > > > +
> > > > if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & DEQUEUE_SAVE))))
> > > > util_est_dequeue(&rq->cfs, p);
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