[PATCH] perf_events: fix time tracking in samples

[Stephane Eranian <eranian@google.com>]

This patch corrects time tracking in samples. Without this patch
both time_enabled and time_running may be reported as zero when
user asks for PERF_SAMPLE_READ.

You use PERF_SAMPLE_READ when you want to sample the values of
other counters in each sample. Because of multiplexing, it is
necessary to know both time_enable, time_running to be able
to scale counts correctly.

We defer updating timing until we know it is really needed, i.e.,
only when we have PERF_SAMPLE_READ.

With this patch, the libpfm4 example task_smpl now reports
correct counts (shown on 2.4GHz Core 2):

$ task_smpl -p 2400000000 -e unhalted_core_cycles:u,instructions_retired:u,baclears  noploop 5
noploop for 5 seconds
IIP:0x000000004006d6 PID:5596 TID:5596 TIME:466,210,211,430 STREAM_ID:33 PERIOD:2,400,000,000 ENA=1,010,157,814 RUN=1,010,157,814 NR=3
	2,400,000,254 unhalted_core_cycles:u (33)
	2,399,273,744 instructions_retired:u (34)
	53,340 baclears (35)

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

---

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index f309e80..04611dd 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -3494,6 +3494,9 @@ static void perf_output_read_group(struct perf_output_handle *handle,
 static void perf_output_read(struct perf_output_handle *handle,
 			     struct perf_event *event)
 {
+	update_context_time(event->ctx);
+	update_event_times(event);
+
 	if (event->attr.read_format & PERF_FORMAT_GROUP)
 		perf_output_read_group(handle, event);
 	else