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Message-ID: <20230927075333.GA210077@ziqianlu-dell>
Date: Wed, 27 Sep 2023 15:53:33 +0800
From: Aaron Lu <aaron.lu@...el.com>
To: Chen Yu <yu.c.chen@...el.com>
CC: Peter Zijlstra <peterz@...radead.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Ingo Molnar <mingo@...hat.com>,
"Vincent Guittot" <vincent.guittot@...aro.org>,
Juri Lelli <juri.lelli@...hat.com>,
"Tim Chen" <tim.c.chen@...el.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>,
"Mel Gorman" <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
K Prateek Nayak <kprateek.nayak@....com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>,
<linux-kernel@...r.kernel.org>, Chen Yu <yu.chen.surf@...il.com>
Subject: Re: [PATCH 1/2] sched/fair: Record the short sleeping time of a task
On Tue, Sep 26, 2023 at 01:11:02PM +0800, Chen Yu wrote:
> During task wakeup, the wakee firstly checks if its previous
> running CPU is idle. If yes, choose that CPU as its first
> choice. However, in most cases, the wakee's previous CPU
> could be chosen by someone else, which breaks the cache
> locality.
>
> Proposes a mechanism to reserve the task's previous
> CPU for a short while. In this reservation period, other
> tasks are not allowed to pick that CPU until a timeout.
> The reservation period is defined as the average short
> sleep time of the task. To be more specific, it is the
> time delta between this task being dequeued and enqueued.
> Only the sleep time shorter than sysctl_sched_migration_cost
> will be recorded. If the sleep time is longer than
> sysctl_sched_migration_cost, give the reservation period
> a penalty by shrinking it to half. In this way, the 'burst'
> sleeping time of the task is honored, meanwhile, if that
> task becomes a long-sleeper, the reservation time of that
> task is shrunk to reduce the impact on task wakeup.
>
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> ---
> include/linux/sched.h | 3 +++
> kernel/sched/fair.c | 21 +++++++++++++++++++++
> 2 files changed, 24 insertions(+)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index dc37ae787e33..4a0ac0276384 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -561,6 +561,9 @@ struct sched_entity {
> u64 vruntime;
> s64 vlag;
> u64 slice;
> + u64 prev_dequeue_time;
> + /* the reservation period of this task during wakeup */
> + u64 sis_rsv_avg;
Nit: these info are only relavant for task, not group so might be better
to move them to task_struct to save a little memory?
>
> u64 nr_migrations;
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d0877878bcdb..297b9470829c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6456,6 +6456,24 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> struct sched_entity *se = &p->se;
> int idle_h_nr_running = task_has_idle_policy(p);
> int task_new = !(flags & ENQUEUE_WAKEUP);
> + u64 last_dequeue = p->se.prev_dequeue_time;
> + u64 now = sched_clock_cpu(task_cpu(p));
I think cpu_of(rq) is more clear than task_cpu(p). Using task_cpu(p)
seems to suggest task_cpu(p) can be different from cpu_of(rq).
> +
> + /*
> + * If the task is a short-sleepting task, there is no need
> + * to migrate it to other CPUs. Estimate the average short sleeping
> + * time of the wakee. This sleep time is used as a hint to reserve
> + * the dequeued task's previous CPU for a short while. During this
> + * reservation period, select_idle_cpu() prevents other wakees from
> + * choosing this CPU. This could bring a better cache locality.
> + */
> + if ((flags & ENQUEUE_WAKEUP) && last_dequeue && cpu_online(task_cpu(p)) &&
Hmm...the cpu_online() check looks weird. If the cpu is offlined, no task
will be enqueued there, right?
Thanks,
Aaron
> + now > last_dequeue) {
> + if (now - last_dequeue < sysctl_sched_migration_cost)
> + update_avg(&p->se.sis_rsv_avg, now - last_dequeue);
> + else
> + p->se.sis_rsv_avg >>= 1;
> + }
>
> /*
> * The code below (indirectly) updates schedutil which looks at
> @@ -6550,6 +6568,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> int task_sleep = flags & DEQUEUE_SLEEP;
> int idle_h_nr_running = task_has_idle_policy(p);
> bool was_sched_idle = sched_idle_rq(rq);
> + u64 now;
>
> util_est_dequeue(&rq->cfs, p);
>
> @@ -6611,6 +6630,8 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> dequeue_throttle:
> util_est_update(&rq->cfs, p, task_sleep);
> hrtick_update(rq);
> + now = sched_clock_cpu(cpu_of(rq));
> + p->se.prev_dequeue_time = task_sleep ? now : 0;
> }
>
> #ifdef CONFIG_SMP
> --
> 2.25.1
>
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