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>

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index b1f8912..cb44c9e 100644 (file)
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -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;

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a355ffb..b3fed52 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -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)