linux-kernel - [tip:perf/urgent] perf/core: Fix ctx_event_type in ctx

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Date:   Fri, 9 Mar 2018 01:07:23 -0800
From:   tip-bot for Song Liu <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        hpa@...or.com, ephiepark@...com, vincent.weaver@...ne.edu,
        songliubraving@...com, alexander.shishkin@...ux.intel.com,
        peterz@...radead.org, kernel-team@...com, jolsa@...hat.com,
        acme@...hat.com, eranian@...gle.com
Subject: [tip:perf/urgent] perf/core: Fix ctx_event_type in ctx_resched()

Commit-ID:  bd903afeb504db5655a45bb4cf86f38be5b1bf62
Gitweb:     https://git.kernel.org/tip/bd903afeb504db5655a45bb4cf86f38be5b1bf62
Author:     Song Liu <songliubraving@...com>
AuthorDate: Mon, 5 Mar 2018 21:55:04 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 9 Mar 2018 08:03:02 +0100

perf/core: Fix ctx_event_type in ctx_resched()

In ctx_resched(), EVENT_FLEXIBLE should be sched_out when EVENT_PINNED is
added. However, ctx_resched() calculates ctx_event_type before checking
this condition. As a result, pinned events will NOT get higher priority
than flexible events.

The following shows this issue on an Intel CPU (where ref-cycles can
only use one hardware counter).

  1. First start:
       perf stat -C 0 -e ref-cycles  -I 1000
  2. Then, in the second console, run:
       perf stat -C 0 -e ref-cycles:D -I 1000

The second perf uses pinned events, which is expected to have higher
priority. However, because it failed in ctx_resched(). It is never
run.

This patch fixes this by calculating ctx_event_type after re-evaluating
event_type.

Reported-by: Ephraim Park <ephiepark@...com>
Signed-off-by: Song Liu <songliubraving@...com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: <jolsa@...hat.com>
Cc: <kernel-team@...com>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: Jiri Olsa <jolsa@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Stephane Eranian <eranian@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vince Weaver <vincent.weaver@...ne.edu>
Fixes: 487f05e18aa4 ("perf/core: Optimize event rescheduling on active contexts")
Link: http://lkml.kernel.org/r/20180306055504.3283731-1-songliubraving@fb.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/events/core.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 96db9ae5d5af..4b838470fac4 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2246,7 +2246,7 @@ static void ctx_resched(struct perf_cpu_context *cpuctx,
 			struct perf_event_context *task_ctx,
 			enum event_type_t event_type)
 {
-	enum event_type_t ctx_event_type = event_type & EVENT_ALL;
+	enum event_type_t ctx_event_type;
 	bool cpu_event = !!(event_type & EVENT_CPU);
 
 	/*
@@ -2256,6 +2256,8 @@ static void ctx_resched(struct perf_cpu_context *cpuctx,
 	if (event_type & EVENT_PINNED)
 		event_type |= EVENT_FLEXIBLE;
 
+	ctx_event_type = event_type & EVENT_ALL;
+
 	perf_pmu_disable(cpuctx->ctx.pmu);
 	if (task_ctx)
 		task_ctx_sched_out(cpuctx, task_ctx, event_type);