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Date: Tue, 5 Jul 2011 15:10:05 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>, Stephane Eranian <eranian@google.com>,
        Lin Ming <ming.m.lin@intel.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH -tip, final] perf, x86: Add hw_watchdog_set_attr() in a
 sake of nmi-watchdog on P4
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* Cyrill Gorcunov <gorcunov@gmail.com> wrote:

> perf, x86: P4 PMU - Add hw_watchdog_set_attr helper to simulate cpu-cycles counting in nmi-watchdog
> 
> Because of constraints existed in Netburst PMU counting
> cpu cycles is allowed for one consumer only.
> 
> If the kernel is booted up with nmi-watchdog enabled
> the watchdog become a consumer of such event and there
> is no more room left for "perf top" and friends (ie any
> attempts to count cpu cycles simultaneously with nmi-watchdog
> doomed to fail).

Hm, what is the symptom - 'perf top' reports nothing?

If multiple users request cycles then perf will time-share them - 
this is what happens if you run many 'perf top' or 'perf stat' 
sessions in parallel for example. For example i just tried to run six 
separate 'perf top' in parallel - and all six worked fine.

So the question is, why does the NMI watchdog prevent 'perf top' from 
working on a P4?

Thanks,

	Ingo