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

Peter Zijlstra
2018-08-03T16:56:41Z

On Wed, Aug 01, 2018 at 11:19:57AM -0400, Johannes Weiner wrote:
> +static bool test_state(unsigned int *tasks, int cpu, enum psi_states state)
> +{
> + switch (state) {
> + case PSI_IO_SOME:
> + return tasks[NR_IOWAIT];
> + case PSI_IO_FULL:
> + return tasks[NR_IOWAIT] && !tasks[NR_RUNNING];
> + case PSI_MEM_SOME:
> + return tasks[NR_MEMSTALL];
> + case PSI_MEM_FULL:
> + /*
> + * Since we care about lost potential, things are
> + * fully blocked on memory when there are no other
> + * working tasks, but also when the CPU is actively
> + * being used by a reclaimer and nothing productive
> + * could run even if it were runnable.
> + */
> + return tasks[NR_MEMSTALL] &&
> + (!tasks[NR_RUNNING] ||
> + cpu_curr(cpu)->flags & PF_MEMSTALL);

I don't think you can do this, there is nothing that guarantees
cpu_curr() still exists.

> + case PSI_CPU_SOME:
> + return tasks[NR_RUNNING] > 1;
> + case PSI_NONIDLE:
> + return tasks[NR_IOWAIT] || tasks[NR_MEMSTALL] ||
> + tasks[NR_RUNNING];
> + default:
> + return false;
> + }
> +}
> +
> +static bool psi_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.
> + *
> + * We don't need to synchronize against CPU hotplugging. If we
> + * see a CPU that's online and has samples, we incorporate it.
> + */
> + for_each_online_cpu(cpu) {
> + struct psi_group_cpu *groupc = per_cpu_ptr(group->pcpu, cpu);
> + u32 uninitialized_var(nonidle);

urgh.. I can see why the compiler got confused. Dodgy :-)

> +
> + BUILD_BUG_ON(PSI_NONIDLE != NR_PSI_STATES - 1);
> +
> + for (s = PSI_NONIDLE; s >= 0; s--) {
> + u32 time, delta;
> +
> + 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;
> + }

The alternative is adding an update to scheduler_tick(), that would
ensure you're never more than nr_cpu_ids * TICK_NSEC behind.

> + delta = time - groupc->times_prev[s];
> + groupc->times_prev[s] = time;
> +
> + if (s == PSI_NONIDLE) {
> + nonidle = nsecs_to_jiffies(delta);
> + nonidle_total += nonidle;
> + } else {
> + deltas[s] += (u64)delta * nonidle;
> + }
> + }
> + }