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    SubjectRe: [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;
    > +}

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