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

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Message-ID: <20180803170733.GC2494@hirez.programming.kicks-ass.net>
Date:   Fri, 3 Aug 2018 19:07:33 +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>,
        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 Wed, Aug 01, 2018 at 11:19:57AM -0400, Johannes Weiner wrote:
> +static bool psi_update_stats(struct psi_group *group)
> +{
> +	u64 deltas[NR_PSI_STATES - 1] = { 0, };
> +	unsigned long missed_periods = 0;
> +	unsigned long nonidle_total = 0;
> +	u64 now, expires, period;
> +	int cpu;
> +	int s;
> +
> +	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 don't need to synchronize against CPU hotplugging. If we
> +	 * see a CPU that's online and has samples, we incorporate it.
> +	 */
> +	for_each_online_cpu(cpu) {

I'm still puzzled by this.. for 99% of the machines online == possible.
Why not always iterate possible and leave it at that? This is hardly a
fast path.