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Date:   Tue, 22 Nov 2022 14:08:19 +0800
From:   Aaron Lu <aaron.lu@intel.com>
To:     Josh Don <joshdon@google.com>
CC:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        "Mel Gorman" <mgorman@suse.de>,
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        Valentin Schneider <vschneid@redhat.com>,
        <linux-kernel@vger.kernel.org>, Tejun Heo <tj@kernel.org>,
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        Christian Brauner <brauner@kernel.org>,
        Zefan Li <lizefan.x@bytedance.com>
Subject: Re: [PATCH v3] sched: async unthrottling for cfs bandwidth
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On Fri, Nov 18, 2022 at 11:25:09AM -0800, Josh Don wrote:
> On Fri, Nov 18, 2022 at 4:47 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > preempt_disable() -- through rq->lock -- also holds off rcu. Strictly
> > speaking this here is superfluous. But if you want it as an annotation,
> > that's fine I suppose.
> 
> Yep, I purely added this as extra annotation for future readers.
> 
> > Ideally we'd first queue all the remotes and then process local, but
> > given how all this is organized that doesn't seem trivial to arrange.
> >
> > Maybe have this function return false when local and save that cfs_rq in
> > a local var to process again later, dunno, that might turn messy.
> 
> Maybe something like this? Apologies for inline diff formatting.
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 012ec9d03811..100dae6023da 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5520,12 +5520,15 @@ static bool distribute_cfs_runtime(struct
> cfs_bandwidth *cfs_b)
>         struct cfs_rq *cfs_rq;
>         u64 runtime, remaining = 1;
>         bool throttled = false;
> +       int this_cpu = smp_processor_id();
> +       struct cfs_rq *local_unthrottle = NULL;
> +       struct rq *rq;
> +       struct rq_flags rf;
> 
>         rcu_read_lock();
>         list_for_each_entry_rcu(cfs_rq, &cfs_b->throttled_cfs_rq,
>                                 throttled_list) {
> -               struct rq *rq = rq_of(cfs_rq);
> -               struct rq_flags rf;
> +               rq = rq_of(cfs_rq);
> 
>                 if (!remaining) {
>                         throttled = true;
> @@ -5556,14 +5559,36 @@ static bool distribute_cfs_runtime(struct
> cfs_bandwidth *cfs_b)
>                 cfs_rq->runtime_remaining += runtime;
> 
>                 /* we check whether we're throttled above */
> -               if (cfs_rq->runtime_remaining > 0)
> -                       unthrottle_cfs_rq_async(cfs_rq);
> +               if (cfs_rq->runtime_remaining > 0) {
> +                       if (cpu_of(rq) != this_cpu ||
> +                           SCHED_WARN_ON(local_unthrottle)) {
> +                               unthrottle_cfs_rq_async(cfs_rq);
> +                       } else {
> +                               local_unthrottle = cfs_rq;
> +                       }
> +               } else {
> +                       throttled = true;
> +               }
> 
>  next:
>                 rq_unlock_irqrestore(rq, &rf);
>         }
>         rcu_read_unlock();
> 
> +       /*
> +        * We prefer to stage the async unthrottles of all the remote cpus
> +        * before we do the inline unthrottle locally. Note that
> +        * unthrottle_cfs_rq_async() on the local cpu is actually synchronous,
> +        * but it includes extra WARNs to make sure the cfs_rq really is
> +        * still throttled.

With this said ->

> +        */
> +       if (local_unthrottle) {
> +               rq = cpu_rq(this_cpu);
> +               rq_lock_irqsave(rq, &rf);

Should we add:
		if (cfs_rq_throttled(local_unthrottle))

before calling into unthrottle_cfs_rq_async(local_unthrottle) to avoid a
potential WARN?

As for whether the local cfs_rq can be unthrottled now after rq lock is
re-acquired, I suppose it can be. e.g. another user sets a new quota to
this task group during the window of rq lock gets dropped in the above
loop and re-acquired here IIUC.

> +               unthrottle_cfs_rq_async(local_unthrottle);
> +               rq_unlock_irqrestore(rq, &rf);
> +       }
> +
>         return throttled;
>  }
>