linux-kernel - Re: [patch] Performance Counters for Linux, v3

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Date:	Mon, 15 Dec 2008 00:13:32 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	eranian@...il.com
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Vince Weaver <vince@...ter.net>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	Robert Richter <robert.richter@....com>,
	Arjan van de Veen <arjan@...radead.org>,
	Peter Anvin <hpa@...or.com>, Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [patch] Performance Counters for Linux, v3


* stephane eranian <eranian@...glemail.com> wrote:

> Hi,
> 
> Given the level of abstractions you are using for the API, and given 
> your argument that the kernel can do the HW resource scheduling better 
> than anybody else.
> 
> What happens in the following test case:
> 
>    - 2-way system (cpu0, cpu1)
> 
>    - on cpu0, two processes P1, P2, each self-monitoring and counting event E1.
>      Event E1 can only be measured on counter C1.
> 
>    - on cpu1, there is a cpu-wide session, monitoring event E1, thus using C1
> 
>    - the scheduler decides to migrate P1 onto CPU1. You now have a
>      conflict on C1.
> 
> How is this managed?

If there's a single unit of sharable resource [such as an event counter, 
or a physical CPU], then there's just three main possibilities: either 
user 1 gets it all, or user 2 gets it all, or they share it.

We've implemented the essence of these variants, with sharing the resource 
being the sane default, and with the sysadmin also having a configuration 
vector to reserve the resource to himself permanently. (There could be 
more variations of this.)

What is your point?

	Ingo
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