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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:
> +/* Tracked task states */
> +enum psi_task_count {
> + NR_RUNNING,
> + NR_IOWAIT,
> + NR_MEMSTALL,
> + NR_PSI_TASK_COUNTS,
> +};
> +
> +/* Task state bitmasks */
> +#define TSK_RUNNING (1 << NR_RUNNING)
> +#define TSK_IOWAIT (1 << NR_IOWAIT)
> +#define TSK_MEMSTALL (1 << NR_MEMSTALL)
> +
> +/* Resources that workloads could be stalled on */
> +enum psi_res {
> + PSI_CPU,
> + PSI_MEM,
> + PSI_IO,
> + NR_PSI_RESOURCES,
> +};
> +
> +/* Pressure states for a group of tasks */
> +enum psi_state {
> + PSI_NONE, /* No stalled tasks */
> + PSI_SOME, /* Stalled tasks & working tasks */
> + PSI_FULL, /* Stalled tasks & no working tasks */
> + NR_PSI_STATES,
> +};
> +
> +struct psi_resource {
> + /* Current pressure state for this resource */
> + enum psi_state state;

This has a 4 byte hole here (really 7 but GCC is generous and uses 4
bytes for the enum that spans the value range [0-2]).

> + /* Start of current state (rq_clock) */
> + u64 state_start;
> +
> + /* Time sampling buckets for pressure states SOME and FULL (ns) */
> + u64 times[2];
> +};
> +
> +struct psi_group_cpu {
> + /* States of the tasks belonging to this group */
> + unsigned int tasks[NR_PSI_TASK_COUNTS];
> +
> + /* There are runnable or D-state tasks */
> + int nonidle;
> +
> + /* Start of current non-idle state (rq_clock) */
> + u64 nonidle_start;
> +
> + /* Time sampling bucket for non-idle state (ns) */
> + u64 nonidle_time;
> +
> + /* Per-resource pressure tracking in this group */
> + struct psi_resource res[NR_PSI_RESOURCES];
> +};

> +static DEFINE_PER_CPU(struct psi_group_cpu, system_group_cpus);

Since psi_group_cpu is exactly 2 lines big, I think you want the above
to be DEFINE_PER_CPU_SHARED_ALIGNED() to minimize cache misses on
accounting. Also, I think you want to stick ____cacheline_aligned_in_smp
on the structure, such that alloc_percpu() also DTRT.

Of those 2 lines, 12 bytes are wasted because of that hole above, and a
further 8 are wasted because PSI_CPU does not use FULL, for a total of
20 wasted bytes in there.

> +static void time_state(struct psi_resource *res, int state, u64 now)
> +{
> + if (res->state != PSI_NONE) {
> + bool was_full = res->state == PSI_FULL;
> +
> + res->times[was_full] += now - res->state_start;
> + }
> + if (res->state != state)
> + res->state = state;
> + if (res->state != PSI_NONE)
> + res->state_start = now;
> +}

Does the compiler optimize that and fold the two != NONE branches?

> +static void psi_group_change(struct psi_group *group, int cpu, u64 now,
> + unsigned int clear, unsigned int set)
> +{
> + enum psi_state state = PSI_NONE;
> + struct psi_group_cpu *groupc;
> + unsigned int *tasks;
> + unsigned int to, bo;
> +
> + groupc = per_cpu_ptr(group->cpus, cpu);
> + tasks = groupc->tasks;

bool was_nonidle = tasks[NR_RUNNING] || tasks[NR_IOWAIT] || tasks[NR_MEMSTALL];

> + /* Update task counts according to the set/clear bitmasks */
> + for (to = 0; (bo = ffs(clear)); to += bo, clear >>= bo) {
> + int idx = to + (bo - 1);
> +
> + if (tasks[idx] == 0 && !psi_bug) {
> + printk_deferred(KERN_ERR "psi: task underflow! cpu=%d idx=%d tasks=[%u %u %u] clear=%x set=%x\n",
> + cpu, idx, tasks[0], tasks[1], tasks[2],
> + clear, set);
> + psi_bug = 1;
> + }

WARN_ONCE(!tasks[idx], ...);

> + tasks[idx]--;
> + }
> + for (to = 0; (bo = ffs(set)); to += bo, set >>= bo)
> + tasks[to + (bo - 1)]++;

You want to benchmark this, but since it's only 3 consecutive bits, it
might actually be faster to not use ffs() and simply test all 3 bits:

for (to = set, bo = 0; to; to &= ~(1 << bo), bo++)
tasks[bo]++;

or something like that.

> +
> + /* Time in which tasks wait for the CPU */
> + state = PSI_NONE;
> + if (tasks[NR_RUNNING] > 1)
> + state = PSI_SOME;
> + time_state(&groupc->res[PSI_CPU], state, now);
> +
> + /* Time in which tasks wait for memory */
> + state = PSI_NONE;
> + if (tasks[NR_MEMSTALL]) {
> + if (!tasks[NR_RUNNING] ||
> + (cpu_curr(cpu)->flags & PF_MEMSTALL))

I'm confused, why do we care if the current tasks is MEMSTALL or not?

> + state = PSI_FULL;
> + else
> + state = PSI_SOME;
> + }
> + time_state(&groupc->res[PSI_MEM], state, now);
> +
> + /* Time in which tasks wait for IO */
> + state = PSI_NONE;
> + if (tasks[NR_IOWAIT]) {
> + if (!tasks[NR_RUNNING])
> + state = PSI_FULL;
> + else
> + state = PSI_SOME;
> + }
> + time_state(&groupc->res[PSI_IO], state, now);
> +
> + /* Time in which tasks are non-idle, to weigh the CPU in summaries */
if (was_nonidle);
> + groupc->nonidle_time += now - groupc->nonidle_start;

if (tasks[NR_RUNNING] || tasks[NR_IOWAIT] || tasks[NR_MEMSTALL])
> + groupc->nonidle_start = now;

Does away with groupc->nonidle, giving us 24 bytes free.

> + /* Kick the stats aggregation worker if it's gone to sleep */
> + if (!delayed_work_pending(&group->clock_work))
> + schedule_delayed_work(&group->clock_work, PSI_FREQ);
> +}

If you always update the time buckets, rename nonidle_start as last_time
and do away with psi_resource::state_start, you gain another 24 bytes,
giving 48 bytes free.

And as said before, we can compress the state from 12 bytes, to 6 bits
(or 1 byte), giving another 11 bytes for 59 bytes free.

Leaving us just 5 bytes short of needing a single cacheline :/

struct ponies {
unsigned int tasks[3]; /* 0 12 */
unsigned int cpu_state:2; /* 12:30 4 */
unsigned int io_state:2; /* 12:28 4 */
unsigned int mem_state:2; /* 12:26 4 */

/* XXX 26 bits hole, try to pack */

/* typedef u64 */ long long unsigned int last_time; /* 16 8 */
/* typedef u64 */ long long unsigned int some_time[3]; /* 24 24 */
/* typedef u64 */ long long unsigned int full_time[2]; /* 48 16 */
/* --- cacheline 1 boundary (64 bytes) --- */
/* typedef u64 */ long long unsigned int nonidle_time; /* 64 8 */

/* size: 72, cachelines: 2, members: 8 */
/* bit holes: 1, sum bit holes: 26 bits */
/* last cacheline: 8 bytes */
};

ARGGH!

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