linux-kernel - Re: x86's nmi_hz wrt. oprofile's nmi_timer

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Message-ID: <20090203122732.GF19979@elte.hu>
Date:	Tue, 3 Feb 2009 13:27:32 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	tglx@...utronix.de, mingo@...hat.com, linux-kernel@...r.kernel.org,
	hpa@...or.com
Subject: Re: x86's nmi_hz wrt. oprofile's nmi_timer_int.c


* David Miller <davem@...emloft.net> wrote:

> From: David Miller <davem@...emloft.net>
> Date: Fri, 30 Jan 2009 13:54:09 -0800 (PST)
> 
> > From: Ingo Molnar <mingo@...e.hu>
> > Date: Fri, 30 Jan 2009 16:01:25 +0100
> > 
> > > 
> > > * David Miller <davem@...emloft.net> wrote:
> > > 
> > > Reducing it to 1 HZ was kind of a performance hack: running NMIs at HZ 
> > > needlessly interrupts the CPU HZ times a second. It's more than enough to 
> > > have 1 nmi-watchdog tick per second to notice deadlocks that take longer 
> > > than 5 seconds.
> > 
> > For the NMI watchdog's purposes I understand the intent, and this
> > is perfectly fine.
> > 
> > The problem is that it stays at '1' when oprofile starts using the NMI
> > watchdog, and we certainly want more than one oprofile tick per second
> > :-)
> 
> Just making sure you understand the problem, here is the
> sequence of events:
> 
> 1) At bootup, the NMI watchdog is tested.
> 
>    It is tested with nmi_hz=HZ
> 
> 2) If the test passes, nmi_hz is reduced down to '1'
> 
> As I stated, everything up to this point is fine.  Next:
> 
> 3) oprofile initializes and if we choose to use the NMI
>    timer for oprofile profiling it is implemented using
>    a simple DIE_NMI notifier.
> 
>    However, nmi_hz is still just '1' which means that oprofile
>    will only receive one sample per-second.  And this is definitely
>    not what we want.
> 
> Somehow the code in arch/x86/oprofile/nmi_timer_int.c needs to have an 
> interface into the NMI watchdog core so that it can increase nmi_hz back 
> up to "HZ" when the NMI timer profiling is enabled and back down to "1" 
> when such profiling stops.

btw., these types of interactions will be solved in a natural way via 
perfcounters: in that model the NMI watchdog is a set of per-CPU counters 
running on each CPU [with a NMI watchdog callback in the IRQ handling 
routine] - and oprofile uses its own perfcounter - what is left of the PMU 
hardware.

I.e. each PMU using facility can just use performance counters transparently 
and interactions will be solved naturally by perfcounters resource 
management.

	Ingo
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