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perf: Avoid a useless pmu_disable() in the perf-tick 

Gleb writes:

 > Currently pmu is disabled and re-enabled on each timer interrupt even
 > when no rotation or frequency adjustment is needed. On Intel CPU this
 > results in two writes into PERF_GLOBAL_CTRL MSR per tick. On bare metal
 > it does not cause significant slowdown, but when running perf in a virtual
 > machine it leads to 20% slowdown on my machine.

Cure this by keeping a perf_event_context::nr_freq counter that counts the
number of active events that require frequency adjustments and use this in a
similar fashion to the already existing nr_events != nr_active test in
perf_rotate_context().

By being able to exclude both rotation and frequency adjustments a-priory for
the common case we can avoid the otherwise superfluous PMU disable.

Suggested-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-515yhoatehd3gza7we9fapaa@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2011-11-16 14:38:16 +01:00 committed by Ingo Molnar
parent d6c1c49de5
commit 0f5a260128
2 changed files with 33 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
include/linux/perf_event.h
View File

@ -890,6 +890,7 @@ struct perf_event_context {
int nr_active;
int is_active;
int nr_stat;
int nr_freq;
int rotate_disable;
atomic_t refcount;
struct task_struct *task;

48
kernel/events/core.c
View File

@ -1130,6 +1130,8 @@ event_sched_out(struct perf_event *event,
if (!is_software_event(event))
cpuctx->active_oncpu--;
ctx->nr_active--;
if (event->attr.freq && event->attr.sample_freq)
ctx->nr_freq--;
if (event->attr.exclusive || !cpuctx->active_oncpu)
cpuctx->exclusive = 0;
}
@ -1407,6 +1409,8 @@ event_sched_in(struct perf_event *event,
if (!is_software_event(event))
cpuctx->active_oncpu++;
ctx->nr_active++;
if (event->attr.freq && event->attr.sample_freq)
ctx->nr_freq++;
if (event->attr.exclusive)
cpuctx->exclusive = 1;
@ -2329,6 +2333,9 @@ static void perf_ctx_adjust_freq(struct perf_event_context *ctx, u64 period)
u64 interrupts, now;
s64 delta;
if (!ctx->nr_freq)
return;
list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
if (event->state != PERF_EVENT_STATE_ACTIVE)
continue;
@ -2384,12 +2391,14 @@ static void perf_rotate_context(struct perf_cpu_context *cpuctx)
{
u64 interval = (u64)cpuctx->jiffies_interval * TICK_NSEC;
struct perf_event_context *ctx = NULL;
int rotate = 0, remove = 1;
int rotate = 0, remove = 1, freq = 0;
if (cpuctx->ctx.nr_events) {
remove = 0;
if (cpuctx->ctx.nr_events != cpuctx->ctx.nr_active)
rotate = 1;
if (cpuctx->ctx.nr_freq)
freq = 1;
}
ctx = cpuctx->task_ctx;
@ -2397,33 +2406,40 @@ static void perf_rotate_context(struct perf_cpu_context *cpuctx)
remove = 0;
if (ctx->nr_events != ctx->nr_active)
rotate = 1;
if (ctx->nr_freq)
freq = 1;
}
if (!rotate && !freq)
goto done;
perf_ctx_lock(cpuctx, cpuctx->task_ctx);
perf_pmu_disable(cpuctx->ctx.pmu);
perf_ctx_adjust_freq(&cpuctx->ctx, interval);
if (ctx)
perf_ctx_adjust_freq(ctx, interval);
if (!rotate)
goto done;
if (freq) {
perf_ctx_adjust_freq(&cpuctx->ctx, interval);
if (ctx)
perf_ctx_adjust_freq(ctx, interval);
}
cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
if (ctx)
ctx_sched_out(ctx, cpuctx, EVENT_FLEXIBLE);
if (rotate) {
cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
if (ctx)
ctx_sched_out(ctx, cpuctx, EVENT_FLEXIBLE);
rotate_ctx(&cpuctx->ctx);
if (ctx)
rotate_ctx(ctx);
rotate_ctx(&cpuctx->ctx);
if (ctx)
rotate_ctx(ctx);
perf_event_sched_in(cpuctx, ctx, current);
perf_event_sched_in(cpuctx, ctx, current);
}
perf_pmu_enable(cpuctx->ctx.pmu);
perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
done:
if (remove)
list_del_init(&cpuctx->rotation_list);
perf_pmu_enable(cpuctx->ctx.pmu);
perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
}
void perf_event_task_tick(void)