linux-kernel - Re: [RFC perf,x86] P4 PMU early draft

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Message-ID: <20100209041739.GA11280@elte.hu>
Date:	Tue, 9 Feb 2010 05:17:39 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Stephane Eranian <eranian@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Don Zickus <dzickus@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC perf,x86] P4 PMU early draft

* Cyrill Gorcunov <gorcunov@...il.com> wrote:

> Hi all,
> 
> first of all the patches are NOT for any kind of inclusion. It's not ready 
> yet. More likely I'm asking for glance review, ideas, criticism.

A quick question: does the code produce something on a real P4? (possibly 
only running with a single event - but even that would be enough.)

> The main problem in implementing P4 PMU is that it has much more 
> restrictions for event to MSR mapping. [...]

One possibly simpler approach might be to represent the P4 PMU via a maximum 
_two_ generic events only.

Last i looked at the many P4 events, i've noticed that generally you can 
create any two events. (with a few exceptions) Once you start trying to take 
advantage of the more than a dozen seemingly separate counters, additional 
non-trivial constraints apply.

So if we only allowed a maximum of _two_ generic events (like say a simple 
Core2 has, so it's not a big restriction at all), we wouldnt have to map all 
the constraints, we'd only have to encode the specific event-to-MSR details. 
(which alone is quite a bit of work as well.)

We could also use the new constraints code to map them all, of course - it 
will certainly be more complex to implement.

Hm?

	Ingo
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