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

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Message-ID: <20150520135209.GI18673@twins.programming.kicks-ass.net>
Date:	Wed, 20 May 2015 15:52:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vince Weaver <vincent.weaver@...ne.edu>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: perf: odd event scheduling issue

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?

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