linux-kernel - Re: NMI watchdog + NOHZ question

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Message-Id: <20090624.033233.170094460.davem@davemloft.net>
Date:	Wed, 24 Jun 2009 03:32:33 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	andi@...stfloor.org
Cc:	linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: NMI watchdog + NOHZ question

From: Andi Kleen <andi@...stfloor.org>
Date: Wed, 24 Jun 2009 12:23:25 +0200

>> And similarly to sparc64, if that 5+ second qla2xxx interrupt
>> sequence happens after the tick_nohz_stop_sched_tick() call
>> we can run into the same situation.
> 
> Yes it would be probably safer to do the tick disabling with
> interrupts off already.

That only makes sense if you're really putting the cpu to sleep
until an interrupt or similar happens.

Here in this sparc64 case I'm not, I just spin waiting for the exit
from cpu_idle() conditions.

I'll think more about how I'll handle this.  It's at least a relief to
understand exactly what causes this issue now :-)

> These days NMI watchdog is not used much on x86 anymore because it's 
> default off, so probably people never noticed that.

I really didn't want to provide the feature that way on sparc64 which
is why I made it on by default.  It would be interesting to reconsider
x86's default, perhaps even only on a trial basis in -next.

It's so useful, and in the short time sparc64 has had this NMI code I
can count at least 8 bugs I've fixed only because it was on all the
time.
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