linux-kernel - Re: [RFC][PATCH 08/11] perf, x86: Implement simple LBR support

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Message-ID: <bd4cb8901003040954n5b05e139wa24f8e50789421ac@mail.gmail.com>
Date:	Thu, 4 Mar 2010 09:54:04 -0800
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org, paulus@...ba.org,
	robert.richter@....com, fweisbec@...il.com
Subject: Re: [RFC][PATCH 08/11] perf, x86: Implement simple LBR support

On Thu, Mar 4, 2010 at 12:58 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, 2010-03-03 at 22:57 +0100, Stephane Eranian wrote:
>> I don't understand how LBR state is migrated when a per-thread event is moved
>> from one CPU to another. It seems LBR is managed per-cpu.
>>
>> Can you explain this to me?
>
> It is not, its basically impossible to do given that the TOS doesn't
> count more bits than is strictly needed.
>
I don't get that about the TOS.

So you are saying that one context switch out, you drop the current
content of LBR. When you are scheduled back in on an another CPU,
you grab whatever is there?

> Or we should stop supporting cpu and task users at the same time.
>
Or you should consider LBR as an event which has a constraint that
it can only run on one pseudo counter (similar to what you do with
BTS). Scheduling would take care of the mutual exclusion. Multiplexing
would provide the work-around.
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