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

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Date:   Tue, 17 Jul 2018 17:01:42 +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 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?

> +	 */
> +	for_each_online_cpu(cpu) {
> +		struct psi_group_cpu *groupc = per_cpu_ptr(group->cpus, cpu);
> +		unsigned long nonidle;
> +
> +		if (!groupc->nonidle_time)
> +			continue;
> +
> +		nonidle = nsecs_to_jiffies(groupc->nonidle_time);
> +		groupc->nonidle_time = 0;
> +		nonidle_total += nonidle;
> +
> +		for (r = 0; r < NR_PSI_RESOURCES; r++) {
> +			struct psi_resource *res = &groupc->res[r];
> +
> +			some[r] += (res->times[0] + res->times[1]) * nonidle;
> +			full[r] += res->times[1] * nonidle;
> +
> +			/* It's racy, but we can tolerate some error */
> +			res->times[0] = 0;
> +			res->times[1] = 0;
> +		}
> +	}
> +
> +	/*
> +	 * Integrate the sample into the running statistics that are
> +	 * reported to userspace: the cumulative stall times and the
> +	 * decaying averages.
> +	 *
> +	 * Pressure percentages are sampled at PSI_FREQ. We might be
> +	 * called more often when the user polls more frequently than
> +	 * that; we might be called less often when there is no task
> +	 * activity, thus no data, and clock ticks are sporadic. The
> +	 * below handles both.
> +	 */
> +
> +	/* 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.

> +	}
> +
> +	/* avgX= */
> +	expires = group->period_expires;
> +	if (time_before(jiffies, expires))
> +		goto out;
> +
> +	missed_periods = (jiffies - expires) / PSI_FREQ;
> +	group->period_expires = expires + ((1 + missed_periods) * PSI_FREQ);
> +
> +	for (r = 0; r < NR_PSI_RESOURCES; r++) {
> +		u64 some, full;
> +
> +		some = group->some[r] - group->last_some[r];
> +		full = group->full[r] - group->last_full[r];
> +
> +		calc_avgs(group->avg_some[r], some, missed_periods);
> +		calc_avgs(group->avg_full[r], full, missed_periods);
> +
> +		group->last_some[r] = group->some[r];
> +		group->last_full[r] = group->full[r];
> +	}
> +out:
> +	mutex_unlock(&group->stat_lock);
> +	return nonidle_total;
> +}