linux-kernel - Re: [PATCH 1/2] perf-events: Add support for supplementary event registers v2

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Date:	Fri, 12 Nov 2010 18:33:12 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	cjashfor@...ux.vnet.ibm.com, mingo@...e.hu, fweisbec@...il.com,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/2] perf-events: Add support for supplementary event
 registers v2

On Fri, 2010-11-12 at 18:17 +0100, Stephane Eranian wrote:
> I looked at this patch thinking how could this be reused for LBR_SELECT.
> 
> I am wondering if storing the extra MSR value in attr.config is really the way
> to go now as opposed to adding/overloading a field.
> 
> For OFFCORE_RESPONSE, it makes sense to use attr.config because this is
> a refinement of the event (a sub-sub event if you want).

Correct, it makes sense for offcore and load-latency, not so much for
lbr_config.

> For LBR_SELECT, you also need to pass a value, but there is no specific event
> associated with it. You can enable LBR with any event you want. Thus,
> storing the
> LBR_SELECT value independently would also make sense. And if we have this
> field for LBR_SELECT then we may as well use it for OFFCORE_REPONSE.

That would assume a single event doesn't contain offcore and lbr, no?
Currently the extra_reg thing assumes there's only one extra reg encoded
in the config word.

> The alternative would be to consider LBR_SELECT also as a refinement of
> the event being measured. Though by itself, it wouldn't do anything, it would
> have to be combine with a PERF_SAMPLE_BRANCH_STACK to attr.sample_type.

A separate field for the lbr_config seems to make most sense, we could
of course use the top 16 bits for lbr, the next 16 for offcore/ll and
the bottom 32 for eventsel, but that's mighty crowded.


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