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Message-ID: <CABPqkBTijzeMSAf5MmynVxS+-Q=Ni_WkygB82OAuR1QSwXyw6g@mail.gmail.com>
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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