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From: Peter Zijlstra <peterz@infradead.org>
To: Vince Weaver <vincent.weaver@maine.edu>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Stephane Eranian <eranian@google.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@redhat.com>, Ingo Molnar <mingo@redhat.com>,
	Paul Mackerras <paulus@samba.org>
Subject: Re: perf: odd event scheduling issue
Date: Wed, 20 May 2015 15:52:09 +0200	[thread overview]
Message-ID: <20150520135209.GI18673@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20150520092842.GF18673@twins.programming.kicks-ass.net>

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?


  reply	other threads:[~2015-05-20 13:52 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-05-20  3:07 perf: odd event scheduling issue Vince Weaver
2015-05-20  7:00 ` Peter Zijlstra
2015-05-20  7:11   ` Jiri Olsa
2015-05-20  9:28   ` Peter Zijlstra
2015-05-20 13:52     ` Peter Zijlstra [this message]
2015-05-20 15:25       ` Peter Zijlstra
2015-05-20 16:08         ` Peter Zijlstra
2015-05-20 16:28           ` Peter Zijlstra
2015-05-20 23:10             ` Andi Kleen
2015-05-21 12:04               ` Stephane Eranian
2015-05-20 16:25       ` Jiri Olsa
2015-05-21 12:05         ` Stephane Eranian

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