[tip:perf/urgent] perf: Fix scaling vs. perf_event_enable_on_exec()

From: tip-bot for Peter Zijlstra
Date: Thu Feb 25 2016 - 03:07:48 EST

Commit-ID: 7fce250915efca0f8f51dddee3ae89bf30d86ca5
Gitweb: http://git.kernel.org/tip/7fce250915efca0f8f51dddee3ae89bf30d86ca5
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Wed, 24 Feb 2016 18:45:48 +0100
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 25 Feb 2016 08:43:34 +0100

perf: Fix scaling vs. perf_event_enable_on_exec()

The recent commit 3e349507d12d ("perf: Fix perf_enable_on_exec() event
scheduling") caused this by moving task_ctx_sched_out() from before
__perf_event_mask_enable() to after it.

The overlooked consequence of that change is that task_ctx_sched_out()
would update the ctx time fields, and now __perf_event_mask_enable()
uses stale time.

In order to fix this, explicitly stop our context's time before
enabling the event(s).

Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: dvyukov@xxxxxxxxxx
Cc: eranian@xxxxxxxxxx
Cc: panand@xxxxxxxxxx
Cc: sasha.levin@xxxxxxxxxx
Cc: vince@xxxxxxxxxx
Fixes: 3e349507d12d ("perf: Fix perf_enable_on_exec() event scheduling")
Link: http://lkml.kernel.org/r/20160224174948.159242158@xxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/events/core.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 75bde93..d003088 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3143,6 +3143,7 @@ static void perf_event_enable_on_exec(int ctxn)

cpuctx = __get_cpu_context(ctx);
perf_ctx_lock(cpuctx, ctx);
+ ctx_sched_out(ctx, cpuctx, EVENT_TIME);
list_for_each_entry(event, &ctx->event_list, event_entry)
enabled |= event_enable_on_exec(event, ctx);