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Message-ID: <Y3xnUhjSb56ex9XX@ziqianlu-desk2>
Date: Tue, 22 Nov 2022 14:08:19 +0800
From: Aaron Lu <aaron.lu@...el.com>
To: Josh Don <joshdon@...gle.com>
CC: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
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>,
<linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
Michal Koutný <mkoutny@...e.com>,
Christian Brauner <brauner@...nel.org>,
Zefan Li <lizefan.x@...edance.com>
Subject: Re: [PATCH v3] sched: async unthrottling for cfs bandwidth
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@...radead.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;
> }
>
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