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Fri, 23 May 2025 02:42:17 -0700 (PDT) Date: Fri, 23 May 2025 17:42:05 +0800 From: Aaron Lu To: Chengming Zhou Cc: Valentin Schneider , Ben Segall , K Prateek Nayak , Peter Zijlstra , Josh Don , Ingo Molnar , Vincent Guittot , Xi Wang , linux-kernel@vger.kernel.org, Juri Lelli , Dietmar Eggemann , Steven Rostedt , Mel Gorman , Chuyi Zhou , Jan Kiszka , Florian Bezdeka Subject: Re: [PATCH 4/7] sched/fair: Take care of group/affinity/sched_class change for throttled task Message-ID: <20250523094106.GA1210419@bytedance> References: <20250520104110.3673059-1-ziqianlu@bytedance.com> <20250520104110.3673059-5-ziqianlu@bytedance.com> <63237b23-ae10-45f9-abdd-8ea4adb4d15e@linux.dev> <20250523075640.GA1168183@bytedance> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 > > > > --- > > > > 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);