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

Johannes Weiner
2018-09-07T14:44:22Z

On Fri, Sep 07, 2018 at 12:16:34PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 28, 2018 at 01:22:57PM -0400, Johannes Weiner wrote:
> > +enum psi_states {
> > + PSI_IO_SOME,
> > + PSI_IO_FULL,
> > + PSI_MEM_SOME,
> > + PSI_MEM_FULL,
> > + PSI_CPU_SOME,
> > + /* Only per-CPU, to weigh the CPU in the global average: */
> > + PSI_NONIDLE,
> > + NR_PSI_STATES,
> > +};
>
> > +static u32 get_recent_time(struct psi_group *group, int cpu,
> > + enum psi_states state)
> > +{
> > + struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
> > + unsigned int seq;
> > + u32 time, delta;
> > +
> > + do {
> > + seq = read_seqcount_begin(&groupc->seq);
> > +
> > + time = groupc->times[state];
> > + /*
> > + * In addition to already concluded states, we also
> > + * incorporate currently active states on the CPU,
> > + * since states may last for many sampling periods.
> > + *
> > + * This way we keep our delta sampling buckets small
> > + * (u32) and our reported pressure close to what's
> > + * actually happening.
> > + */
> > + if (test_state(groupc->tasks, state))
> > + time += cpu_clock(cpu) - groupc->state_start;
> > + } while (read_seqcount_retry(&groupc->seq, seq));
> > +
> > + delta = time - groupc->times_prev[state];
> > + groupc->times_prev[state] = time;
> > +
> > + return delta;
> > +}
>
> > +static bool 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.
> > + */
> > + for_each_possible_cpu(cpu) {
> > + u32 nonidle;
> > +
> > + nonidle = get_recent_time(group, cpu, PSI_NONIDLE);
> > + nonidle = nsecs_to_jiffies(nonidle);
> > + nonidle_total += nonidle;
> > +
> > + for (s = 0; s < PSI_NONIDLE; s++) {
> > + u32 delta;
> > +
> > + delta = get_recent_time(group, cpu, s);
> > + deltas[s] += (u64)delta * nonidle;
> > + }
> > + }
>
> This does the whole seqcount thing 6x, which is a bit of a waste.

[...]

> It's a bit cumbersome, but that's because of C.

I was actually debating exactly this with Suren before, but since this
is a super cold path I went with readability. I was also thinking that
restarts could happen quite regularly under heavy scheduler load, and
so keeping the individual retry sections small could be helpful - but
I didn't instrument this in any way.

No strong opinion from me, I can send an updated patch if you prefer.