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Message-ID: <871cf7c1-951c-4ddd-9800-db96e050c6d9@linux.dev>
Date: Wed, 21 May 2025 17:01:58 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Aaron Lu <ziqianlu@...edance.com>,
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>
Cc: 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 2/7] sched/fair: prepare throttle path for task based
throttle
On 2025/5/20 18:41, Aaron Lu wrote:
> From: Valentin Schneider <vschneid@...hat.com>
>
> In current throttle model, when a cfs_rq is throttled, its entity will
> be dequeued from cpu's rq, making tasks attached to it not able to run,
> thus achiveing the throttle target.
>
> This has a drawback though: assume a task is a reader of percpu_rwsem
> and is waiting. When it gets wakeup, it can not run till its task group's
> next period comes, which can be a relatively long time. Waiting writer
> will have to wait longer due to this and it also makes further reader
> build up and eventually trigger task hung.
>
> To improve this situation, change the throttle model to task based, i.e.
> when a cfs_rq is throttled, record its throttled status but do not remove
> it from cpu's rq. Instead, for tasks that belong to this cfs_rq, when
> they get picked, add a task work to them so that when they return
> to user, they can be dequeued. In this way, tasks throttled will not
> hold any kernel resources.
>
> To avoid breaking bisect, preserve the current throttle behavior by
> still dequeuing throttled hierarchy from rq and because of this, no task
> can have that throttle task work added yet. The throttle model will
> switch to task based in a later patch.
>
> Suggested-by: Chengming Zhou <chengming.zhou@...ux.dev> # tag on pick
> Signed-off-by: Valentin Schneider <vschneid@...hat.com>
> Signed-off-by: Aaron Lu <ziqianlu@...edance.com>
I'm wondering how about put 02-04 patches together, since it's strange
to setup task work in this patch but without changing throttle_cfs_rq(),
which makes the reviewing process a bit confused? WDYT?
Thanks!
> ---
> kernel/sched/fair.c | 88 +++++++++++++++++++++++++++++++++++++++-----
> kernel/sched/sched.h | 1 +
> 2 files changed, 80 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 75bf6186a5137..e87ceb0a2d37f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5825,8 +5825,47 @@ static inline int throttled_lb_pair(struct task_group *tg,
> throttled_hierarchy(dest_cfs_rq);
> }
>
> +static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags);
> static void throttle_cfs_rq_work(struct callback_head *work)
> {
> + struct task_struct *p = container_of(work, struct task_struct, sched_throttle_work);
> + struct sched_entity *se;
> + struct cfs_rq *cfs_rq;
> + struct rq *rq;
> +
> + WARN_ON_ONCE(p != current);
> + p->sched_throttle_work.next = &p->sched_throttle_work;
> +
> + /*
> + * If task is exiting, then there won't be a return to userspace, so we
> + * don't have to bother with any of this.
> + */
> + if ((p->flags & PF_EXITING))
> + return;
> +
> + scoped_guard(task_rq_lock, p) {
> + se = &p->se;
> + cfs_rq = cfs_rq_of(se);
> +
> + /* Raced, forget */
> + if (p->sched_class != &fair_sched_class)
> + return;
> +
> + /*
> + * If not in limbo, then either replenish has happened or this
> + * task got migrated out of the throttled cfs_rq, move along.
> + */
> + if (!cfs_rq->throttle_count)
> + return;
> + rq = scope.rq;
> + update_rq_clock(rq);
> + WARN_ON_ONCE(!list_empty(&p->throttle_node));
> + dequeue_task_fair(rq, p, DEQUEUE_SLEEP | DEQUEUE_SPECIAL);
> + list_add(&p->throttle_node, &cfs_rq->throttled_limbo_list);
> + resched_curr(rq);
> + }
> +
> + cond_resched_tasks_rcu_qs();
> }
>
> void init_cfs_throttle_work(struct task_struct *p)
> @@ -5866,21 +5905,42 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
> return 0;
> }
>
> +static inline bool task_has_throttle_work(struct task_struct *p)
> +{
> + return p->sched_throttle_work.next != &p->sched_throttle_work;
> +}
> +
> +static inline void task_throttle_setup_work(struct task_struct *p)
> +{
> + if (task_has_throttle_work(p))
> + return;
> +
> + /*
> + * Kthreads and exiting tasks don't return to userspace, so adding the
> + * work is pointless
> + */
> + if ((p->flags & (PF_EXITING | PF_KTHREAD)))
> + return;
> +
> + task_work_add(p, &p->sched_throttle_work, TWA_RESUME);
> +}
> +
> static int tg_throttle_down(struct task_group *tg, void *data)
> {
> struct rq *rq = data;
> struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
>
> + cfs_rq->throttle_count++;
> + if (cfs_rq->throttle_count > 1)
> + return 0;
> +
> /* group is entering throttled state, stop time */
> - if (!cfs_rq->throttle_count) {
> - cfs_rq->throttled_clock_pelt = rq_clock_pelt(rq);
> - list_del_leaf_cfs_rq(cfs_rq);
> + cfs_rq->throttled_clock_pelt = rq_clock_pelt(rq);
> + list_del_leaf_cfs_rq(cfs_rq);
>
> - WARN_ON_ONCE(cfs_rq->throttled_clock_self);
> - if (cfs_rq->nr_queued)
> - cfs_rq->throttled_clock_self = rq_clock(rq);
> - }
> - cfs_rq->throttle_count++;
> + WARN_ON_ONCE(cfs_rq->throttled_clock_self);
> + if (cfs_rq->nr_queued)
> + cfs_rq->throttled_clock_self = rq_clock(rq);
>
> return 0;
> }
> @@ -6575,6 +6635,7 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
> cfs_rq->runtime_enabled = 0;
> INIT_LIST_HEAD(&cfs_rq->throttled_list);
> INIT_LIST_HEAD(&cfs_rq->throttled_csd_list);
> + INIT_LIST_HEAD(&cfs_rq->throttled_limbo_list);
> }
>
> void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> @@ -6744,6 +6805,7 @@ static bool check_cfs_rq_runtime(struct cfs_rq *cfs_rq) { return false; }
> static void check_enqueue_throttle(struct cfs_rq *cfs_rq) {}
> 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 inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
> {
> @@ -8851,6 +8913,7 @@ static struct task_struct *pick_task_fair(struct rq *rq)
> {
> struct sched_entity *se;
> struct cfs_rq *cfs_rq;
> + struct task_struct *p;
>
> again:
> cfs_rq = &rq->cfs;
> @@ -8871,7 +8934,14 @@ static struct task_struct *pick_task_fair(struct rq *rq)
> cfs_rq = group_cfs_rq(se);
> } while (cfs_rq);
>
> - return task_of(se);
> + p = task_of(se);
> + if (throttled_hierarchy(cfs_rq_of(se))) {
> + /* Shuold not happen for now */
> + WARN_ON_ONCE(1);
> + task_throttle_setup_work(p);
> + }
> +
> + return p;
> }
>
> static void __set_next_task_fair(struct rq *rq, struct task_struct *p, bool first);
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 921527327f107..83f16fc44884f 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -736,6 +736,7 @@ struct cfs_rq {
> int throttle_count;
> struct list_head throttled_list;
> struct list_head throttled_csd_list;
> + struct list_head throttled_limbo_list;
> #endif /* CONFIG_CFS_BANDWIDTH */
> #endif /* CONFIG_FAIR_GROUP_SCHED */
> };
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