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Subject[tip:perf/core] perf/cgroups: Don't rotate events for cgroups unnecessarily
Commit-ID:  fd7d55172d1e2e501e6da0a5c1de25f06612dc2e
Gitweb: https://git.kernel.org/tip/fd7d55172d1e2e501e6da0a5c1de25f06612dc2e
Author: Ian Rogers <irogers@google.com>
AuthorDate: Sat, 1 Jun 2019 01:27:22 -0700
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 24 Jun 2019 19:30:04 +0200

perf/cgroups: Don't rotate events for cgroups unnecessarily

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 <irogers@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: https://lkml.kernel.org/r/20190601082722.44543-1-irogers@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
include/linux/perf_event.h | 5 +++++
kernel/events/core.c | 42 ++++++++++++++++++++++--------------------
2 files changed, 27 insertions(+), 20 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 3dc01cf98e16..2ddae518dce6 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -749,6 +749,11 @@ struct perf_event_context {
int nr_stat;
int nr_freq;
int rotate_disable;
+ /*
+ * Set when nr_events != nr_active, except tolerant to events not
+ * necessary to be active due to scheduling constraints, such as cgroups.
+ */
+ int rotate_necessary;
refcount_t refcount;
struct task_struct *task;

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 118ad1aef6af..23efe6792abc 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)
@@ -3319,10 +3325,13 @@ static int flexible_sched_in(struct perf_event *event, void *data)
return 0;

if (group_can_go_on(event, sid->cpuctx, sid->can_add_hw)) {
- if (!group_sched_in(event, sid->cpuctx, sid->ctx))
- list_add_tail(&event->active_list, &sid->ctx->flexible_active);
- else
+ int ret = group_sched_in(event, sid->cpuctx, sid->ctx);
+ if (ret) {
sid->can_add_hw = 0;
+ sid->ctx->rotate_necessary = 1;
+ return 0;
+ }
+ list_add_tail(&event->active_list, &sid->ctx->flexible_active);
}

return 0;
@@ -3690,24 +3699,17 @@ 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 +3718,7 @@ 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 +3726,17 @@ 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);
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