linux-kernel - Re: [PATCH 1/1] perf tools: Add missing user space support for config1/config2

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Date:	Mon, 25 Apr 2011 15:12:58 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Vince Weaver <vweaver1@...s.utk.edu>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...il.com>,
	Lin Ming <ming.m.lin@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 1/1] perf tools: Add missing user space support for
 config1/config2

> The PAPI tool was using the perf_events interface in the 2.6.39-git 
> kernels to collect offcore response results by properly setting the 
> config1 register on Nehalem and Westmere machines.

I already had some users for this functionality too. Offcore
events are quite useful for various analysis: basically every time
you have a memory performance problem -- especially a NUMA
problem -- they can help you a lot tracking it down.

They answer questions like "who accesses memory on another node"

As far as I'm concerned b52c55c6a25e4515b5e075a989ff346fc251ed09 
is a bad feature regression.

> 
> Now it has been disabled for unclear reasons.

Also unfortunately only partial. Previously you could at least
write the MSR from user space through /dev/cpu/*/msr, but now the kernel 
randomly rewrites it if anyone else uses cache events.

Right now I have some frontend scripts which are doing this,
but it's really quite nasty.

It's very sad we have to go through this.

-Andi

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