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

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Date:	Fri, 30 Jan 2009 16:01:25 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	tglx@...utronix.de, mingo@...hat.com, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: x86's nmi_hz wrt. oprofile's nmi_timer_int.c


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

> While working on an NMI watchdog implementation on sparc64 I noticed 
> what seems to be a peculiar behavior of the NMI timer int oprofile 
> support on x86.
> 
> When the NMI watchdog tests itself at boot timer we start with nmi_hz 
> equal to HZ.
> 
> After the NMI watchdog self-test passes, nmi_hz is reduced down to '1'.
> 
> The NMI timer int oprofile support simply uses DIE_NMI notifiers for 
> it's implementation.  But I don't see anything in the code of 
> arch/x86/oprofile/nmi_timer_int.c nor the NMI watchdog infrastructure 
> which will re-adjust nmi_hz back to HZ or something similar.
> 
> Am I missing something?

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.

Can you see a problem with that approach, or was this just a question 
about why it's reduced to 1 Hz?

	Ingo
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