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

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Date:   Wed, 18 Jul 2018 18:06:23 -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>,
        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 Tue, Jul 17, 2018 at 05:01:42PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 12, 2018 at 01:29:40PM -0400, Johannes Weiner wrote:
> > +static bool psi_update_stats(struct psi_group *group)
> > +{
> > +	u64 some[NR_PSI_RESOURCES] = { 0, };
> > +	u64 full[NR_PSI_RESOURCES] = { 0, };
> > +	unsigned long nonidle_total = 0;
> > +	unsigned long missed_periods;
> > +	unsigned long expires;
> > +	int cpu;
> > +	int r;
> > +
> > +	mutex_lock(&group->stat_lock);
> > +
> > +	/*
> > +	 * Collect the per-cpu time buckets and average them into a
> > +	 * single time sample that is normalized to wallclock time.
> > +	 *
> > +	 * For averaging, each CPU is weighted by its non-idle time in
> > +	 * the sampling period. This eliminates artifacts from uneven
> > +	 * loading, or even entirely idle CPUs.
> > +	 *
> > +	 * We could pin the online CPUs here, but the noise introduced
> > +	 * by missing up to one sample period from CPUs that are going
> > +	 * away shouldn't matter in practice - just like the noise of
> > +	 * previously offlined CPUs returning with a non-zero sample.
> 
> But why!? cpuu_read_lock() is neither expensive nor complicated. So why
> try and avoid it?

Hm, I don't feel strongly about it either way. I'll add it.

> > +	/* total= */
> > +	for (r = 0; r < NR_PSI_RESOURCES; r++) {
> > +		do_div(some[r], max(nonidle_total, 1UL));
> > +		do_div(full[r], max(nonidle_total, 1UL));
> > +
> > +		group->some[r] += some[r];
> > +		group->full[r] += full[r];
> 
> 		group->some[r] = div64_ul(some[r], max(nonidle_total, 1UL));
> 		group->full[r] = div64_ul(full[r], max(nonidle_total, 1UL));
> 
> Is easier to read imo.

Sounds good to me, I'll change that.