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Date:   Fri, 15 May 2020 15:12:39 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Jirka Hladky <jhladky@redhat.com>, Phil Auld <pauld@redhat.com>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Hillf Danton <hdanton@sina.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Douglas Shakshober <dshaks@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Joe Mario <jmario@redhat.com>, Bill Gray <bgray@redhat.com>
Subject: Re: [PATCH 00/13] Reconcile NUMA balancing decisions with the load
 balancer v6
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On Fri, May 15, 2020 at 02:03:46PM +0100, Mel Gorman wrote:
> On Fri, May 15, 2020 at 01:17:32PM +0200, Peter Zijlstra wrote:
> > On Fri, May 15, 2020 at 09:47:40AM +0100, Mel Gorman wrote:
> > +static bool ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
> > +{
> > +	if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {
> > +		sched_clock_cpu(cpu); /* Sync clocks across CPUs */
> > +		__ttwu_queue_remote(p, cpu, wake_flags);
> > +		return true;
> > +	}
> > +
> > +	return false;
> > +}

> > +	if (READ_ONCE(p->on_cpu) && __ttwu_queue_remote(p, cpu, wake_flags))
> > +		goto unlock;

> I don't see a problem with moving the updating of p->state to the other
> side of the barrier but I'm relying on the comment that the barrier is
> only related to on_rq and on_cpu.

Yeah, I went with that too, like I said, didn't think too hard.

> However, I'm less sure about what exactly you intended to do.
> __ttwu_queue_remote is void so maybe you meant to use ttwu_queue_remote.

That!

> In that case, we potentially avoid spinning on on_rq for wakeups between
> tasks that do not share CPU but it's not clear why it would be specific to
> remote tasks.

The thinking was that we can avoid spinning on ->on_cpu, and let the CPU
get on with things. Rik had a workload where that spinning was
significant, and I thought to have understood you saw the same.

By sticking the task on the wake_list of the CPU that's in charge of
clearing ->on_cpu we ensure ->on_cpu is 0 by the time we get to doing
the actual enqueue.