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

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Message-ID: <20090223045159.GR26292@one.firstfloor.org>
Date:	Mon, 23 Feb 2009 05:52:00 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	David Miller <davem@...emloft.net>
Cc:	andi@...stfloor.org, linux-kernel@...r.kernel.org
Subject: Re: x86's nmi_hz wrt. oprofile's nmi_timer_int.c

> Again, the code in nmi_timer_int.c doesn't.
> 
> It uses the NMI watchdog timer interrupts, it catches DIE_NMI
> events.
> 
> > Does that answer your question?
> 
> Not really.

Ah see what you mean now. The nmi_timer_int code can only
be active ever when the cpu is not known to nmi_int.c and 
when the nmi watchdog is in io apic mode. But IO apic mode
doesn't use the fast check/slowdown because it always runs
at HZ frequency. That only happens in LAPIC mode.

The standard fallback mode for unknown CPU is the non NMI timer 
fallback in oprofile_init, the IO APIC mode happens near never in practice.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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