perf_events: Fix bogus context time tracking

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@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: stable@kernel.org
LKML-Reference: <4cb856dc.51edd80a.5ae0.38fb@mx.google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>