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

From: Liang, Kan
Date: Fri Jun 14 2019 - 17:48:17 EST



On 6/14/2019 3:10 PM, Stephane Eranian wrote:
On Thu, Jun 13, 2019 at 9:13 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:



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 <irogers@xxxxxxxxxx>
---
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?

I am not sure I see the point here. What I'd like to see is a uniform
signal for rotation needed in per-task, per-cpu or per-cgroup modes > Ian's patch does that. It does make it a lot more efficient in cgroup
mode, by avoiding unnecessary rotations, and does not alter/improve
on any of the other two modes.

I just thought if it is only used by cgroup, it may be better to move it under #ifdef CONFIG_CGROUP_PERF. For users who don't care about cgroup, it may save some space.
But if it's designed as a uniform signal, this is OK.

Thanks,
Kan


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);