Re: [BUG] perf_events: PERF_FORMAT_GROUP not working correctly whenmonitoring another task
From: Corey Ashford
Date: Thu May 06 2010 - 22:40:34 EST
On 05/06/2010 01:22 PM, Peter Zijlstra wrote:
On Thu, 2010-05-06 at 13:09 -0700, Corey Ashford wrote:
The patch works with the test case I provided, but for some reason it
breaks the normal operation of the libpfm4 "task" utility. If I put
more than one event in a group, I get zero counts on all but the first
event. That's even if I don't use the PERF_FORMAT_GROUP option.
Bugger,.. weird though, I can't see how it would affect the state before
the counters get detached from their context (exit() / close()).
So something appears to be messed up. I will see if I can construct an
arch-independent test case which demonstrates the problem.
That would be awesome, thanks!
Ok, it was pilot error. We had made a few changes to the "task" program
to expand its capabilities and introduced a couple of bugs at the same time.
Unfortunately, I have one remaining mystery. The default behavior of
task is to create groups, without the PERF_FORMAT_GROUP flag set, and
then read up the counts and time enabled/running values for each counter
in the group after the measurement has occurred.
Well, for some reason I haven't been able to figure out, all but first
event in the group have their time running and time enable values set to
zero, but their counts are non-zero.
If I replicate this setup with the show_fg_bug test case I posted, the
time running and enabled values are correct for each counter.
The enabled and running flag bits are set for every event which is
opened (in both show_fg_bug and task)... so I don't know what the
difference would be.
If anything springs to mind there as to how I might be getting zero
enabled/running values passed back from the kernel, I'd be very
interested to know.
Otherwise, I will keep poking at it till I find the bug in task, or
something I can point at in the kernel.
Thanks,
- Corey
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