linux-kernel - Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO

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Date:   Mon, 6 Aug 2018 11:40:51 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Peter Zijlstra <peterz@...radead.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>,
        Daniel Drake <drake@...lessm.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Mike Galbraith <efault@....de>,
        Shakeel Butt <shakeelb@...gle.com>,
        Peter Enderborg <peter.enderborg@...y.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and
 IO

On Mon, Aug 06, 2018 at 05:25:28PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 06, 2018 at 11:05:50AM -0400, Johannes Weiner wrote:
> > Argh, that's right. This needs an explicit count if we want to access
> > it locklessly. And you already said you didn't like that this is the
> > only state not derived purely from the task counters, so maybe this is
> > the way to go after all.
> > 
> > How about something like this (untested)?
> 
> 
> > +static inline void psi_switch(struct rq *rq, struct task_struct *prev,
> > +			      struct task_struct *next)
> > +{
> > +	if (psi_disabled)
> > +		return;
> > +
> > +	if (unlikely(prev->flags & PF_MEMSTALL))
> > +		psi_task_change(prev, rq_clock(rq), TSK_RECLAIMING, 0);
> > +	if (unlikely(next->flags & PF_MEMSTALL))
> > +		psi_task_change(next, rq_clock(rq), 0, TSK_RECLAIMING);
> > +}
> 
> 
> Urgh... can't say I really like that.
> 
> I would really rather do that scheduler_tick() thing to avoid the remote
> update. The tick is a lot less hot than the switch path and esp.
> next->flags might be a cold line (prev->flags is typically the same line
> as prev->state so we already have that, but I don't think anybody now
> looks at next->flags or its line, so that'd be cold load).

Okay, the tick updater sounds like a much better option then. HZ
frequency should produce more than recent enough data.

That means we will retain the not-so-nice PF_MEMSTALL flag test under
rq lock, but it'll eliminate most of that memory ordering headache.

I'll do that. Thanks!