Funny perf

[Borislav Petkov]

Hi guys,

I've been playing with latest perf (tip/perf/core) and I'm hitting the
warning in perf_session_queue_event() "Warning: Timestamp below last
timeslice flush" which means IMHO that the timestamps of each sample are
not monotonically increasing, as expected. But here's what I do, it could very
well be that I'm doing it wrong:

$ perf probe sched_clock_tick
$ perf record -f -e probe:sched_clock_tick -a -g ./nbench
$ perf report -g --stdio

Warning:...

The interesting thing is that dropping the '-a', i.e. "all-cpus" option
from perf record doesn't trigger the warning anymore.

Then, I added some silly debug output to see what happens:

diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index a3a871f..79c7184 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -622,6 +622,7 @@ static int perf_session_queue_event(struct perf_session *s, union perf_event *ev
 
        if (timestamp < s->ordered_samples.last_flush) {
                printf("Warning: Timestamp below last timeslice flush\n");
+               printf("ts: %lu <-> %lu (last_flush), cpu: %d\n", timestamp, s->ordered_samples.last_flush, sample->cpu)
                return -EINVAL;
        }

Result:

...

Warning: Timestamp below last timeslice flush
ts: 10304235947415373581 <-> 18446744069414589173 (last_flush), cpu: -8321639
Warning: Timestamp below last timeslice flush
ts: 72057594029606669 <-> 18446744069414589173 (last_flush), cpu: 0
Warning: Timestamp below last timeslice flush
ts: 18446744069414584320 <-> 18446744069414589173 (last_flush), cpu: 0
Warning: Timestamp below last timeslice flush
ts: 6052837899177642039 <-> 18446744069414589173 (last_flush), cpu: -8323025
Warning: Timestamp below last timeslice flush
ts: 8502796096467174809 <-> 18446744069414589173 (last_flush), cpu: -8321142
Warning: Timestamp below last timeslice flush
ts: 648518346358128639 <-> 18446744069414589173 (last_flush), cpu: 16777216
Warning: Timestamp below last timeslice flush
ts: 11 <-> 18446744069414589173 (last_flush), cpu: -128
Warning: Timestamp below last timeslice flush
ts: 4755801206503243774 <-> 18446744069414589173 (last_flush), cpu: 16441
Warning: Timestamp below last timeslice flush
ts: 4294967295 <-> 18446744069414589173 (last_flush), cpu: 9
Warning: Timestamp below last timeslice flush
ts: 17654401922121072640 <-> 18446744069414589173 (last_flush), cpu: -16777198
Warning: Timestamp below last timeslice flush
ts: 2017612633061998649 <-> 18446744069414589173 (last_flush), cpu: -637534208
...

so this looks like some of the samples do not belong in the perf.data
output with that garbled cpu field. It lets me also assume that the
sample->time field is also crap so the timestamp check is falsely
tripped.

But this is all I got, now you can tell me how much I suck :).

Thanks.

-- 
Regards/Gruss,
Boris.

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