linux-kernel - Re: [PATCH V7 13/17] perf, x86: enable LBR callstack when recording callchain

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Message-ID: <20141105174031.GO3274@tassilo.jf.intel.com>
Date:	Wed, 5 Nov 2014 09:40:31 -0800
From:	Andi Kleen <ak@...ux.intel.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Kan Liang <kan.liang@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCH V7 13/17] perf, x86: enable LBR callstack when recording
 callchain

> The reason why using LBR call stack mode is restricted to user level
> only is because of
> a bug in the LBR call stack hardware which forces the kernel to drop
> LBR_FREEZE_PMI.

It works with PEBS events, just not with non PEBS (the patch currently
does not implement this distinction and I'm not sure it's worth it)

> In other words ,the LBR does not stop on counter overflow, thus it
> will be wiped out by
> execution of kernel code leading to the PMU interrupt handler.

It stops, but it takes some time to stop so the call-return stack
can get out of sync.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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