linux-kernel - [PATCH 9/8] perf_events: Restore sanity to scaling land

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Message-ID: <1258984836.4531.480.camel@laptop>
Date:	Mon, 23 Nov 2009 15:00:36 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: [PATCH 9/8] perf_events: Restore sanity to scaling land

It is quite possible to call update_event_times() on a context that
isn't actually running and thereby confuse the thing.

perf stat was reporting !100% scale values for software counters
(2e2af50b perf_events: Disable events when we detach them, solved the
worst of that, but there was still some left).

The thing that happens is that because we are not self-reaping (we have
a caring parent) there is a time between the last schedule (out) and
having do_exit() called which will detach the events.

This period would be accounted as enabled,!running because the
event->state==INACTIVE, even though !event->ctx->is_active.

Similar issues could have been observed by calling read() on a event
while the attached task was not scheduled in.

Solve this by teaching update_event_times() about ctx->is_active.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
 kernel/perf_event.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 0b0d5f7..0aafe85 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -274,7 +274,12 @@ static void update_event_times(struct perf_event *event)
 	    event->group_leader->state < PERF_EVENT_STATE_INACTIVE)
 		return;

-	event->total_time_enabled = ctx->time - event->tstamp_enabled;
+	if (ctx->is_active)
+		run_end = ctx->time;
+	else
+		run_end = event->tstamp_stopped;
+
+	event->total_time_enabled = run_end - event->tstamp_enabled;

 	if (event->state == PERF_EVENT_STATE_INACTIVE)
 		run_end = event->tstamp_stopped;

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