linux-kernel - Re: [PATCH v4] Sched/fair: Block nohz tick

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Date:   Fri, 30 Jun 2023 12:29:10 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Ben Segall <bsegall@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v4] Sched/fair: Block nohz tick_stop when cfs bandwidth
 in use

On Fri, Jun 30, 2023 at 06:05:34PM +0200 Peter Zijlstra wrote:
> On Fri, Jun 30, 2023 at 11:28:24AM -0400, Phil Auld wrote:
> 
> > No. Or at least not without plumbing the enqueued/dequeued task all the way
> > through.  I can do it that way if you prefer but that seemed a lot more
> > intrusive. When we are in sched_can_stop_tick() we don't have access to the
> > cfs task which will end up running. Curr is idle in that case.  We'd have to
> > essential run pick_next_task_fair() to find the task to check which seemed
> > wrong. Maybe there is a better way?
> 
> Ah, you worry about where we have two runnable tasks, one is bandwidth
> constrained the other is not. One task goes away, how can we tell what
> the remaining task is?

It didn't even have to be an unconstrained task before I added the
check in sched_can_stop_tick(). But yes that check catches the case
where the one (still) running task is bw constrained.

> 
> This is never a concern for add_nr_running(), the only case there is
> 0->1 and then only the hierarchy you just walked for enqueue is
> relevant.
>

Right but we don't (currently) have the task here (hence the pick_next code).

> But if you remove the unconstrained task, sub_nr_running() can't tell
> what the remaining task is.
> 
> Unless, of course, you have enqueue() set a bit somewhere in
> task_struct::sched_bw_constrained:1.
> 
> Then pick and your should_stop thing can look at that, no?
> 

Yes.

I think you are agreeing that I need the pick next code but need to remove
the hierarchy walks, right?

If I set the bit in task_struct it would do that. I'll go poke at that. 


Thanks,
Phil

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