Date:   Fri, 18 Nov 2022 11:25:09 -0800
From:   Josh Don <joshdon@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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 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.
+        */
+       if (local_unthrottle) {
+               rq = cpu_rq(this_cpu);
+               rq_lock_irqsave(rq, &rf);
+               unthrottle_cfs_rq_async(local_unthrottle);
+               rq_unlock_irqrestore(rq, &rf);
+       }
+
        return throttled;
 }

Note that one change we definitely want is the extra setting of
throttled = true in the case that cfs_rq->runtime_remaining <= 0, to
catch the case where we run out of runtime to distribute on the last
entity in the list.

> > +
> > +     /* Already enqueued */
> > +     if (SCHED_WARN_ON(!list_empty(&cfs_rq->throttled_csd_list)))
> > +             return;
> > +
> > +     list_add_tail(&cfs_rq->throttled_csd_list, &rq->cfsb_csd_list);
> > +
> > +     smp_call_function_single_async(cpu_of(rq), &rq->cfsb_csd);
>
> Hurmph.. so I was expecting something like:
>
>         first = list_empty(&rq->cfsb_csd_list);
>         list_add_tail(&cfs_rq->throttled_csd_list, &rq->cfsb_csd_list);
>         if (first)
>                 smp_call_function_single_async(cpu_of(rq), &rq->cfsb_csd);
>
> But I suppose I'm remembering the 'old' version. I don't think it is
> broken as written. There's a very narrow window where you'll end up
> sending a second IPI for naught, but meh.

The CSD doesn't get  unlocked until right before we call the func().
But you're right that that's a (very) narrow window for an  extra IPI.
Please feel free to modify the patch with that diff if you like.

>
> > +}
>
> Let me go queue this thing, we can always improve upon matters later.

Thanks! Please add at least the extra assignment of 'throttled = true'
from the diff above, but feel free to squash both the diffs if it
makes sense to you.

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