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 <eran...@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
------------------------------------------------------------------------------
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