Re: [PATCH] perf cgroups: Don't rotate events for cgroups unnecessarily

[Liang, Kan]

On 6/1/2019 4:27 AM, Ian Rogers wrote:
Currently perf_rotate_context assumes that if the context's nr_events !=
nr_active a rotation is necessary for perf event multiplexing. With
cgroups, nr_events is the total count of events for all cgroups and
nr_active will not include events in a cgroup other than the current
task's. This makes rotation appear necessary for cgroups when it is not.

Add a perf_event_context flag that is set when rotation is necessary.
Clear the flag during sched_out and set it when a flexible sched_in
fails due to resources.

Signed-off-by: Ian Rogers <irog...@google.com>
---
  include/linux/perf_event.h |  5 +++++
  kernel/events/core.c       | 42 +++++++++++++++++++++++---------------
  2 files changed, 30 insertions(+), 17 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 15a82ff0aefe..7ab6c251aa3d 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -747,6 +747,11 @@ struct perf_event_context {
        int                             nr_stat;
        int                             nr_freq;
        int                             rotate_disable;
+       /*
+        * Set when nr_events != nr_active, except tolerant to events not
+        * needing to be active due to scheduling constraints, such as cgroups.
+        */
+       int                             rotate_necessary;

It looks like the rotate_necessary is only useful for cgroup and cpuctx.
Why not move it to struct perf_cpu_context and under #ifdef 
CONFIG_CGROUP_PERF?
And rename it cgrp_rotate_necessary?

Thanks,
Kan

        refcount_t                      refcount;
        struct task_struct              *task;
  
diff --git a/kernel/events/core.c b/kernel/events/core.c
index abbd4b3b96c2..41ae424b9f1d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2952,6 +2952,12 @@ static void ctx_sched_out(struct perf_event_context *ctx,
        if (!ctx->nr_active || !(is_active & EVENT_ALL))
                return;
  
+	/*
+        * If we had been multiplexing, no rotations are necessary now no events
+        * are active.
+        */
+       ctx->rotate_necessary = 0;
+
        perf_pmu_disable(ctx->pmu);
        if (is_active & EVENT_PINNED) {
                list_for_each_entry_safe(event, tmp, &ctx->pinned_active, 
active_list)
@@ -3325,6 +3331,15 @@ static int flexible_sched_in(struct perf_event *event, 
void *data)
                        sid->can_add_hw = 0;
        }
  
+	/*
+        * If the group wasn't scheduled then set that multiplexing is necessary
+        * for the context. Note, this won't be set if the event wasn't
+        * scheduled due to event_filter_match failing due to the earlier
+        * return.
+        */
+       if (event->state == PERF_EVENT_STATE_INACTIVE)
+               sid->ctx->rotate_necessary = 1;
+
        return 0;
  }
  
@@ -3690,24 +3705,17 @@ ctx_first_active(struct perf_event_context *ctx)
  static bool perf_rotate_context(struct perf_cpu_context *cpuctx)
  {
        struct perf_event *cpu_event = NULL, *task_event = NULL;
-       bool cpu_rotate = false, task_rotate = false;
-       struct perf_event_context *ctx = NULL;
+       struct perf_event_context *task_ctx = NULL;
+       int cpu_rotate, task_rotate;
  
  	/*
         * Since we run this from IRQ context, nobody can install new
         * events, thus the event count values are stable.
         */
  
-	if (cpuctx->ctx.nr_events) {
-               if (cpuctx->ctx.nr_events != cpuctx->ctx.nr_active)
-                       cpu_rotate = true;
-       }
-
-       ctx = cpuctx->task_ctx;
-       if (ctx && ctx->nr_events) {
-               if (ctx->nr_events != ctx->nr_active)
-                       task_rotate = true;
-       }
+       cpu_rotate = cpuctx->ctx.rotate_necessary;
+       task_ctx = cpuctx->task_ctx;
+       task_rotate = task_ctx ? task_ctx->rotate_necessary : 0;
  
  	if (!(cpu_rotate || task_rotate))
                return false;
@@ -3716,7 +3724,7 @@ static bool perf_rotate_context(struct perf_cpu_context 
*cpuctx)
        perf_pmu_disable(cpuctx->ctx.pmu);
  
  	if (task_rotate)
-               task_event = ctx_first_active(ctx);
+               task_event = ctx_first_active(task_ctx);
        if (cpu_rotate)
                cpu_event = ctx_first_active(&cpuctx->ctx);
  
@@ -3724,17 +3732,17 @@ static bool perf_rotate_context(struct perf_cpu_context *cpuctx)
         * As per the order given at ctx_resched() first 'pop' task flexible
         * and then, if needed CPU flexible.
         */
-       if (task_event || (ctx && cpu_event))
-               ctx_sched_out(ctx, cpuctx, EVENT_FLEXIBLE);
+       if (task_event || (task_ctx && cpu_event))
+               ctx_sched_out(task_ctx, cpuctx, EVENT_FLEXIBLE);
        if (cpu_event)
                cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
  
  	if (task_event)
-               rotate_ctx(ctx, task_event);
+               rotate_ctx(task_ctx, task_event);
        if (cpu_event)
                rotate_ctx(&cpuctx->ctx, cpu_event);
  
-	perf_event_sched_in(cpuctx, ctx, current);
+       perf_event_sched_in(cpuctx, task_ctx, current);
  
  	perf_pmu_enable(cpuctx->ctx.pmu);
        perf_ctx_unlock(cpuctx, cpuctx->task_ctx);