Re: [PATCH 6/7] psi: pressure stall information for CPU, memory, and IO

Suren Baghdasaryan
2018-06-07T00:46:26Z

Hi Johannes,

On Mon, May 7, 2018 at 2:01 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> +static void psi_clock(struct work_struct *work)
> +{
> + u64 some[NR_PSI_RESOURCES] = { 0, };
> + u64 full[NR_PSI_RESOURCES] = { 0, };
> + unsigned long nonidle_total = 0;
> + unsigned long missed_periods;
> + struct delayed_work *dwork;
> + struct psi_group *group;
> + unsigned long expires;
> + int cpu;
> + int r;
> +
> + dwork = to_delayed_work(work);
> + group = container_of(dwork, struct psi_group, clock_work);
> +
> + /*
> + * Calculate the sampling period. The clock might have been
> + * stopped for a while.
> + */
> + expires = group->period_expires;
> + missed_periods = (jiffies - expires) / MY_LOAD_FREQ;
> + group->period_expires = expires + ((1 + missed_periods) * MY_LOAD_FREQ);
> +
> + /*
> + * Aggregate the per-cpu state into a global state. Each CPU
> + * is weighted by its non-idle time in the sampling period.
> + */

Would it be possible to move this aggregation code (excluding
calc_avgs()) into a separate function which is called from here as
well as from psi_show() before group->some[] and group->full[] are
reported? This would not affect the performance if the information is
not requested and at the same time would keep at least the "total"
field up-to-date when the data is requested. For calc_avgs() I think
we would have to calculate the change in nonidle_total, group->some[]
and group->full[] fields differently because a call to psi_show() in
the middle of two psi_clock() calls would refresh these fields before
2secs expire, however calculating that change is trivial if we store
previous group->some[], group->full[] and nonidle_total values inside
psi_clock(). This would require new fields in psi_group struct to
store these previous values but the upside is that we would eliminate
the problem with reporting potentially stale data (up to 2sec update
delay) and provide a function one can use to refresh group->some[] and
group->full[] and implement custom averaging.

> + for_each_online_cpu(cpu) {
> + struct psi_group_cpu *groupc = per_cpu_ptr(group->cpus, cpu);
> + unsigned long nonidle;
> +
> + 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;
> + }
> + }
> +
> + for (r = 0; r < NR_PSI_RESOURCES; r++) {
> + /* Finish the weighted aggregation */
> + some[r] /= max(nonidle_total, 1UL);
> + full[r] /= max(nonidle_total, 1UL);
> +
> + /* Accumulate stall time */
> + group->some[r] += some[r];
> + group->full[r] += full[r];
> +
> + /* Calculate recent pressure averages */
> + calc_avgs(group->avg_some[r], some[r], missed_periods);
> + calc_avgs(group->avg_full[r], full[r], missed_periods);
> + }
> +
> + /* Keep the clock ticking only when there is action */
> + if (nonidle_total)
> + schedule_delayed_work(dwork, MY_LOAD_FREQ);
> +}
> +

Thanks,
Suren.