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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)
> +{

> + 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;
> + }
> + }

An alternative for this, that also allows that ondemand update, but
without spamming the rq->lock would be something like:

struct psi_group_cpu {
u32 tasks[3];
u32 cpu_state : 2;
u32 mem_state : 2;
u32 io_state : 2;
u32 :0;

u64 last_update_time;

u32 nonidle;
u32 full[2];
u32 some[3];
} ____cacheline_aligned_in_smp;

/* Allocate _2_ copies */
DEFINE_PER_CPU_ALIGNED_SHARED(struct psi_group_cpu[2], psi_cpus);

struct psi_group global_psi = {
.cpus = &psi_cpus[0],
};


u64 sums[6] = { 0, };

for_each_possible_cpu(cpu) {
struct psi_group_cpu *pgc = per_cpu_ptr(group->cpus, cpu);
u32 *active, *shadow;

active = &pgc[0].nonidle;
shadow = &pgc[1].nonidle;

/*
* Compare the active count to the shadow count
* if different, compute the delta and update the shadow
* copy.
* This only writes to the shadow copy (separate line)
* and leaves the active a read-only access.
*/
for (i = 0; i < 6; i++) {
u32 old = READ_ONCE(shadow[i]);
u32 new = READ_ONCE(active[i]);

delta = (new - old);
if (!delta) {
if (!i)
goto next;
continue;
}

WRITE_ONCE(shadow[i], new);

sums[i] += delta;
}
next: ;
}

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