linux-kernel - Re: [PATCH] sched, fair: Allow a per-cpu kthread waking a task to stack on the same CPU

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Date:   Thu, 30 Jan 2020 09:06:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Dave Chinner <david@...morbit.com>, Ingo Molnar <mingo@...hat.com>,
        Tejun Heo <tj@...nel.org>, Phil Auld <pauld@...hat.com>,
        Ming Lei <ming.lei@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched, fair: Allow a per-cpu kthread waking a task to
 stack on the same CPU

On Thu, Jan 30, 2020 at 12:43:34AM +0000, Mel Gorman wrote:
> On Wed, Jan 29, 2020 at 06:38:52PM +0100, Peter Zijlstra wrote:

> > I suppose the fact that it limits it to tasks that were running on the
> > same CPU limits the impact if we do get it wrong.
> > 
> 
> And it's limited to no other task currently running on the
> CPU. Now, potentially multiple sleepers are on that CPU waiting for
> a mutex/rwsem/completion but it's very unlikely and mostly likely due
> to the machine being saturated in which case searching for an idle CPU
> will probably fail. It would also be bound by a small window after the
> first wakeup before the task becomes runnable before the nr_running check
> mitigages the problem. Besides, if the sleeping task is waiting on the
> lock, it *is* related to the kworker which is probably finished.
> 
> In other words, even this patches worst-case behaviour does not seem
> that bad.

OK; let's just stick it in and see what, if anything, falls over :-)

I saw there is a v2 out (although I didn't see what changed in a hurry),
let me queue that one.