linux-kernel - Re: [PATCH -tip, final] perf, x86: Add hw_watchdog_set

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Date:	Fri, 8 Jul 2011 14:44:35 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Don Zickus <dzickus@...hat.com>,
	Stephane Eranian <eranian@...gle.com>,
	Lin Ming <ming.m.lin@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -tip, final] perf, x86: Add hw_watchdog_set_attr() in a
 sake of nmi-watchdog on P4


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

> On Tue, Jul 05, 2011 at 03:31:05PM +0200, Ingo Molnar wrote:
> > 
> > * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > 
> > > > So the question is, why does the NMI watchdog prevent 'perf top' 
> > > > from working on a P4?
> > > 
> > > because the NMI watchdog is a pinned event, you don't want to share 
> > > the counter, that would be very bad, suppose you lock up when the 
> > > NMI watchdog was scheduled out. Unreliably debug tools are worse 
> > > than no tools.
> > 
> > Yeah, indeed that explains the symptom.
> > 
> > Firstly, we should fix/enhance perf top to print out an error message 
> > in this case, not just hang there doing nothing.
> 
> It waits for event to be scheduled, so seems first the kernel 
> should have top level context-schedule-in functions changed from 
> void to int (so I have admit I might be missin something here).
> 
> > 
> > Secondly, the proper solution would be to allow the multiplexing of 
> > like-minded hw events. Here if we have two events:
> > 
> >   - pinned NMI watchdog, set to a period of 2 billion cycles
> >   - perf top with a default of 1 khz auto-freq cycles
> > 
> > We should first change the NMI watchdog to use auto-freq samples - 
> > the hw_nmi_get_sample_period() looks unnecessary - if we set the NMI 
> > watchdog to 1 Hz by default it should be more than enough.
> 
> Should we drop tunability of watchdog period as well? At moment
> there is a way to setup period via /sys.

Well, the tunable can remain - there's some real usecases that want 
to increased/decrease the frequency.

Thanks,

	Ingo
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