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

Peter Zijlstra
2018-08-03T17:21:39Z

On Wed, Aug 01, 2018 at 11:19:57AM -0400, Johannes Weiner wrote:
> + time = READ_ONCE(groupc->times[s]);
> + /*
> + * 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, cpu, s)) {
> + /*
> + * We can race with a state change and
> + * need to make sure the state_start
> + * update is ordered against the
> + * updates to the live state and the
> + * time buckets (groupc->times).
> + *
> + * 1. If we observe task state that
> + * needs to be recorded, make sure we
> + * see state_start from when that
> + * state went into effect or we'll
> + * count time from the previous state.
> + *
> + * 2. If the time delta has already
> + * been added to the bucket, make sure
> + * we don't see it in state_start or
> + * we'll count it twice.
> + *
> + * If the time delta is out of
> + * state_start but not in the time
> + * bucket yet, we'll miss it entirely
> + * and handle it in the next period.
> + */
> + smp_rmb();
> + time += cpu_clock(cpu) - groupc->state_start;
> + }

As is, groupc->state_start needs a READ_ONCE() above and a WRITE_ONCE()
below. But like stated earlier, doing an update in scheduler_tick() is
probably easier.

> +static void psi_group_change(struct psi_group *group, int cpu, u64 now,
> + unsigned int clear, unsigned int set)
> +{
> + struct psi_group_cpu *groupc;
> + unsigned int t, m;
> + u32 delta;
> +
> + groupc = per_cpu_ptr(group->pcpu, cpu);
> +
> + /*
> + * First we assess the aggregate resource states these CPU's
> + * tasks have been in since the last change, and account any
> + * SOME and FULL time that may have resulted in.
> + *
> + * Then we update the task counts according to the state
> + * change requested through the @clear and @set bits.
> + */
> +
> + delta = now - groupc->state_start;
> + groupc->state_start = now;
> +
> + /*
> + * Update state_start before recording time in the sampling
> + * buckets and changing task counts, to prevent a racing
> + * aggregation from counting the delta twice or attributing it
> + * to an old state.
> + */
> + smp_wmb();
> +
> + if (test_state(groupc->tasks, cpu, PSI_IO_SOME)) {
> + groupc->times[PSI_IO_SOME] += delta;
> + if (test_state(groupc->tasks, cpu, PSI_IO_FULL))
> + groupc->times[PSI_IO_FULL] += delta;
> + }
> + if (test_state(groupc->tasks, cpu, PSI_MEM_SOME)) {
> + groupc->times[PSI_MEM_SOME] += delta;
> + if (test_state(groupc->tasks, cpu, PSI_MEM_FULL))
> + groupc->times[PSI_MEM_FULL] += delta;
> + }

Might we worth checking the compiler does the right thing here and
optimizes this branch fest into something sensible.

> + if (test_state(groupc->tasks, cpu, PSI_CPU_SOME))
> + groupc->times[PSI_CPU_SOME] += delta;
> + if (test_state(groupc->tasks, cpu, PSI_NONIDLE))
> + groupc->times[PSI_NONIDLE] += delta;