linux-kernel - Re: perf: odd event scheduling issue

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Date:	Thu, 21 May 2015 05:05:26 -0700
From:	Stephane Eranian <eranian@...gle.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: perf: odd event scheduling issue

On Wed, May 20, 2015 at 9:25 AM, Jiri Olsa <jolsa@...hat.com> wrote:
> 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@...gle.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@...gle.com>
>> >     Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>> >     Cc: bp@...en8.de
>> >     Cc: jolsa@...hat.com
>> >     Cc: kan.liang@...el.com
>> >     Cc: maria.n.dimakopoulou@...il.com
>> >     Link: http://lkml.kernel.org/r/1416251225-17721-11-git-send-email-eranian@google.com
>> >     Signed-off-by: Ingo Molnar <mingo@...nel.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
>
But maybe HT was disabled on your snb machine.

> jirka
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