linux-kernel - Re: [PATCH 4/7] perf, x86: Save/resotre LBR stack during context switch

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Message-ID: <20130626114704.GF28407@twins.programming.kicks-ass.net>
Date:	Wed, 26 Jun 2013 13:47:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, eranian@...gle.com,
	andi@...stfloor.org
Subject: Re: [PATCH 4/7] perf, x86: Save/resotre LBR stack during context
 switch

On Tue, Jun 25, 2013 at 04:47:16PM +0800, Yan, Zheng wrote:
> From: "Yan, Zheng" <zheng.z.yan@...el.com>
> 
> When the LBR call stack is enabled, it is necessary to save/restore
> the stack on context switch. The solution is saving/restoring the
> stack to/from task's perf event context. If task has no perf event
> context, just flush the stack on context switch.

So I have some problems with this; suppose the LBR is shared between a
cpu and a task event, when the task switches back in, it will over-write
the LBR content with old stuff.

In that case the CPU event's LBR stack can be actively wrong, as opposed
to being empty.
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