Re: perf: odd event scheduling issue

Jiri Olsa
2015-05-20T16:25:18Z

On Wed, May 20, 2015 at 03:52:09PM +0200, Peter Zijlstra wrote:
> On Wed, May 20, 2015 at 11:28:42AM +0200, Peter Zijlstra wrote:
>
> > ---
> >
> > c02cdbf60b51b8d98a49185535f5d527a2965142 is the first bad commit
> > commit c02cdbf60b51b8d98a49185535f5d527a2965142
> > Author: Stephane Eranian <eranian@google.com>
> > Date: Mon Nov 17 20:07:02 2014 +0100
> >
> > perf/x86/intel: Limit to half counters when the HT workaround is enabled, to avoid exclusive mode starvation
> >
> > This patch limits the number of counters available to each CPU when
> > the HT bug workaround is enabled.
> >
> > This is necessary to avoid situation of counter starvation. Such can
> > arise from configuration where one HT thread, HT0, is using all 4 counters
> > with corrupting events which require exclusion the the sibling HT, HT1.
> >
> > In such case, HT1 would not be able to schedule any event until HT0
> > is done. To mitigate this problem, this patch artificially limits
> > the number of counters to 2.
> >
> > That way, we can gurantee that at least 2 counters are not in exclusive
> > mode and therefore allow the sibling thread to schedule events of the
> > same type (system vs. per-thread). The 2 counters are not determined
> > in advance. We simply set the limit to two events per HT.
> >
> > This helps mitigate starvation in case of events with specific counter
> > constraints such a PREC_DIST.
> >
> > Note that this does not elimintate the starvation is all cases. But
> > it is better than not having it.
> >
> > (Solution suggested by Peter Zjilstra.)
> >
> > Signed-off-by: Stephane Eranian <eranian@google.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: bp@alien8.de
> > Cc: jolsa@redhat.com
> > Cc: kan.liang@intel.com
> > Cc: maria.n.dimakopoulou@gmail.com
> > Link: http://lkml.kernel.org/r/1416251225-17721-11-git-send-email-eranian@google.com
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
>
> OK, so if you have the watchdog enabled, that's 1 event, and having a
> max of 2 GP events, adding another 2 events is fail.
>
> Jiri, did you SNB have the watchdog disabled?

watchdog was enabled

jirka