Messages in this thread | | | Date | Wed, 18 Jul 2018 14:03:18 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 08/10] psi: pressure stall information for CPU, memory, and IO |
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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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