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>

1 files changed, 32 insertions, 16 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a355ffb0b28f..b3fed52aaf20 100644
--- 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)