linux-kernel - Re: [RFC -tip] perf, x86: Add PERF_COUNT_HW_NMI

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Message-ID: <20110621172319.GE21641@sun>
Date:	Tue, 21 Jun 2011 21:23:19 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Don Zickus <dzickus@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lin Ming <ming.m.lin@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Vince Weaver <vweaver1@...s.utk.edu>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC -tip] perf, x86: Add PERF_COUNT_HW_NMI_WATCHDOG event v2

On Tue, Jun 21, 2011 at 07:10:05PM +0200, Stephane Eranian wrote:
...
> >
> >  The key here is that we use that named non-sleeping ticks (as oprofile
> > did) for nmi-watchdog and it allows us to free "cpu-cycles" counter for
> > user needs. Of course we pick up one counter for this but it doesn't intersect
> > with "cpu-cycles" counter (because counters are grouped and can count only
> > specified events in each group). Stepane, should I post more details?
> >
> Ok, so you're using a different PMU event for the watchdog. But then, in this
> case why not simply change kernel/watchdog.c to hardcode something specific
> for P4, i.e, change the wd_hw_attr.config/type fields to match the
> event you want.

  It doesn't change things much. Of course I can put some specifics into
.config but where is the guarantee some new generic event would not ever
intersect with it. I know (for example) we could reserve -1ULL for this
event but again where is the guarantee that it will never ever be used
system wide in future for some different event?

So I need some event-id in which I may be sure it will never be
used for different purpose.

> You could create a arch specific callback to setup config/type.
> 
> I don't think it makes sense to expose yet another generic PMU event, especially
> given the name you gave to it and what it actually does. People might wonder
> what good is that for?

	Cyrill
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