linux-kernel - Re: NMI watchdog + NOHZ question

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Message-Id: <20090624.035914.156829493.davem@davemloft.net>
Date:	Wed, 24 Jun 2009 03:59:14 -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 12:52:23 +0200

> On Wed, Jun 24, 2009 at 03:32:33AM -0700, David Miller wrote:
>> 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.
> 
> That is what the idle loop is supposed to do, isn't it?

Some sparc64 cpu's don't have a yield, and therefore can't
truly "sleep" during this loop.  That's what I'm talking
about.

>> > 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.
> 
> The reason it was turned off is that there are a few systems (e.g.
> laptops from a particular vendor) which don't handle NMIs correctly
> in the platform. When the NMI happens while SMI is active
> they hang. Also there were a few other strange problems
> on other systems that went away when it was disabled.

I wonder how many of those "few other strange problems" were of
the variety I'm diagnosing here :-)

Yes, it's a messy problem to turn on by default on x86 then.

Is this realm of systems-with-NMI-issues exclusive to x86-32 
or would it be more doable to turn it on by default for 64-bit
x86 builds?
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