linux-kernel - Re: [RFC PATCH 2/3 v2] perf: Implement Nehalem uncore pmu

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Date:	Thu, 25 Nov 2010 07:09:10 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC PATCH 2/3 v2] perf: Implement Nehalem uncore pmu

On Thu, 2010-11-25 at 08:24 +0800, Lin Ming wrote:
> On Tue, 2010-11-23 at 18:00 +0800, Stephane Eranian wrote:
> > On Sun, Nov 21, 2010 at 6:44 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > > On Sun, 2010-11-21 at 22:04 +0800, Lin Ming wrote:
> > >> On Sun, 2010-11-21 at 20:46 +0800, Andi Kleen wrote:
> > >> > >
> > >> > > 2. Uncore pmu NMI handling
> > >> > >
> > >> > > All the 4 cores are programmed to receive uncore counter overflow
> > >> > > interrupt. The NMI handler(running on 1 of the 4 cores) handle all
> > >> > > counters enabled by all 4 cores.
> > >> >
> > >> > Really for uncore monitoring there is no need to use an NMI handler.
> > >> > You can't profile a core anyways, so you can just delay the reporting
> > >> > a little bit. It may simplify the code to not use one here
> > >> > and just use an ordinary handler.
> > >>
> > >> OK, I can use on ordinary interrupt handler here.
> > >
> > > Does the hardware actually allow using a different interrupt source?
> > >
> > It does not. It's using whatever you've programmed into the APIC
> > LVT vector, AFAIK. Uncore interrupt mode is enabled via
> > IA32_DEBUGCTL. Regarless of sampling or not, you need the interrupt
> > to virtualize the counters to 64 bits.
> 
> If only counting(perf stat) makes sense for uncore events, do we still
> need an interrupt handler?

Yep, I see no reason to dis-allow sampling. Sure its hard to make sense
of it, but since there are people who offline all but one cpu of a
package, I bet there are people who will run just one task on a package
as well.

Just because it doesn't make sense in general doesn't mean there isn't
anybody who'd want to do it and actually knows wth he's doing.

> 48 bits counter is not that easy to overflow in practice.

Still..
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