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Date:   Mon, 3 Jul 2023 10:10:56 -0400
From:   Phil Auld <pauld@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Ben Segall <bsegall@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH v4] Sched/fair: Block nohz tick_stop when cfs bandwidth
 in use
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On Mon, Jul 03, 2023 at 02:10:09PM +0200 Peter Zijlstra wrote:
> On Fri, Jun 30, 2023 at 12:29:10PM -0400, Phil Auld wrote:
> 
> > I think you are agreeing that I need the pick next code but need to remove
> > the hierarchy walks, right?
> 
> Yeah, the dequeue case makes we have to care about pick, not sure we
> then also need to care about sched_update_tick_dependency() though.
> There is indeed a window where these two will 'race', but afaict it is
> benign.
> 

Hm, that's confusing.

As I see it it's the enqueue case (0->1 mostly) where we need the check
in pick.  At that point in enqueue we only have a handle on ->curr which
is the idle thread.

For the dequeue case (2->1) we need the check in the
sched_update_tick_dependency() path because if the 1 is the task on the
cpu (and is staying there) then we'd otherwise clear the bit when we
shouldn't (since we aren't going to go back through pick).

I'm thinking that I'll try to set the bit in pick since we only care about
it when it's the task on the cpu.  That, I think, will simplify the
code needed to update the bit when the quota is changed (to or from
RUNTIME_INF).

Setting the bit in enqueue/dequeue means updating it on all the queued
task if it changes. Although I may clear it in dequeue just to not leave
it around stale.


Cheers,
Phil
--