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

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Message-Id: <20090222.215906.240407314.davem@davemloft.net>
Date:	Sun, 22 Feb 2009 21:59:06 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	andi@...stfloor.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: x86's nmi_hz wrt. oprofile's nmi_timer_int.c

From: Andi Kleen <andi@...stfloor.org>
Date: Mon, 23 Feb 2009 05:52:00 +0100

> > 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.

Look at the fallback logic, the pure NMI profiler can fail for
a number of reasons, not just because the watchdog is in
I/O APIC mode.

No matter what the failure reason, nmi_int.c is used.

And in some of those cases, the nmi_hz has been decreased to '1'.
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