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

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Date:	Thu, 25 Nov 2010 09:48:46 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Lin Ming <ming.m.lin@...el.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, Nov 25, 2010 at 7:09 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 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..
>
Agreed.
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