linux-kernel - [PATCH] perf_events: fix bogus context time tracking

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Message-ID: <4cb856dc.51edd80a.5ae0.38fb@mx.google.com>
Date:	Fri, 15 Oct 2010 15:26:01 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	linux-kernel@...r.kernel.org
Cc:	peterz@...radead.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com,
	perfmon2-devel@...ts.sf.net, eranian@...il.com, eranian@...gle.com,
	robert.richter@....com
Subject: [PATCH] perf_events: fix bogus context time tracking

You can only call update_context_time() when the context
is active, i.e., the thread it is attached to is still running.

However, perf_event_read() can be called even when the context
is inactive, e.g., user read() the counters. The call to
update_context_time() must be conditioned on the status of
the context, otherwise, bogus time_enabled, time_running may
be returned. Here is an example on AMD64. The task program
is an example from libpfm4. The -p prints deltas every 1s.

$ task -p -e cpu_clk_unhalted sleep 5
    2,266,610 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
	    0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
	    0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
	    0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
	    0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982)
5,242,358,071 cpu_clk_unhalted (99.95% scaling, ena=5,000,359,984, run=2,319,270)

Whereas if you don't read deltas, e.g., no call to perf_event_read() until
the process terminates:

$ task -e cpu_clk_unhalted sleep 5
    2,497,783 cpu_clk_unhalted (0.00% scaling, ena=2,376,899, run=2,376,899)

Notice that time_enable, time_running are bogus in the first example
causing bogus scaling.

This patch fixes the problem, by conditionally calling update_context_time()
in perf_event_read().

Signed-off-by: Stephane Eranian <eranian@...gle.com>

---

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index b6fd9ec..4dae345 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -1775,7 +1775,13 @@ static u64 perf_event_read(struct perf_event *event)
 		unsigned long flags;
 
 		raw_spin_lock_irqsave(&ctx->lock, flags);
-		update_context_time(ctx);
+		/*
+		 * may read while context is not active
+		 * (e.g., thread is blocked), in that case
+		 * we cannot update context time
+		 */
+		if (ctx->is_active)
+			update_context_time(ctx);
 		update_event_times(event);
 		raw_spin_unlock_irqrestore(&ctx->lock, flags);
 	}
--
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