linux-kernel - Re: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO

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Date:   Thu, 19 Jul 2018 15:58:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Mike Galbraith <efault@....de>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 08/10] psi: pressure stall information for CPU, memory,
 and IO

On Wed, Jul 18, 2018 at 06:36:44PM -0400, Johannes Weiner wrote:
> On Wed, Jul 18, 2018 at 02:03:18PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 12, 2018 at 01:29:40PM -0400, Johannes Weiner wrote:
> > > +	/* Time in which tasks wait for the CPU */
> > > +	state = PSI_NONE;
> > > +	if (tasks[NR_RUNNING] > 1)
> > > +		state = PSI_SOME;
> > > +	time_state(&groupc->res[PSI_CPU], state, now);
> > > +
> > > +	/* Time in which tasks wait for memory */
> > > +	state = PSI_NONE;
> > > +	if (tasks[NR_MEMSTALL]) {
> > > +		if (!tasks[NR_RUNNING] ||
> > > +		    (cpu_curr(cpu)->flags & PF_MEMSTALL))
> > 
> > I'm confused, why do we care if the current tasks is MEMSTALL or not?
> 
> We want to know whether we're losing CPU potential because of a lack
> of memory. That can happen when the task waits for refaults and the
> CPU goes idle, but it can also happen when the CPU is performing
> reclaim.
> 
> If the task waits for refaults and something else is runnable, we're
> not losing CPU potential. But if the task performs reclaim and uses
> the CPU, nothing else can do productive work on that CPU.

Right, this is because MEMSTALL is not just blocking (as per that other
sub-thread).

This is really unfortunate, because it means the state is not a simple
function of the task counts.