linux-kernel - Re: [PATCH] x86_64: Avoid NMI Watchdog and/or long wait in setup_APIC

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Date:	Thu, 9 Aug 2007 02:26:23 +0200
From:	Andi Kleen <ak@...e.de>
To:	Aaron Durbin <adurbin@...gle.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...l.org
Subject: Re: [PATCH] x86_64: Avoid NMI Watchdog and/or long wait in setup_APIC_timer

On Thursday 09 August 2007 01:17:19 Aaron Durbin wrote:
> In setup_APIC_timer with the HPET in use, a condition can arise while
> waiting for the next irq slice to expire on the HPET which will either
> cause an NMI watchdog to fire or a 3 minute busy loop if the NMI
> watchdog is disabled.
> 
> The HPET comparator and the counter keep incrementing during its normal
> operation. When a comparison event fires the comparator will increment
> by the designated period. If the HPET trigger occurs right after
> the 'int trigger = hpet_readl(HPET_T0_CMP);' line, we will will spin
> for up to 3 minutes (with a clock of 25MHz) waiting for the HPET
> counter to wrap around. However, when the NMI watchdog is enabled the
> NMI watchdog will detect a lockup and reboot the machine. This
> scenario can be exasperated by the presence of an SMI which will
> increase the window of opportunity for the condition to occur.

Nasty.

Iirc the clockevents code did away with the synchronization
completely because Thomas determined it was useless because long term
it'll drift anyways. So perhaps it could be just removed?

-Andi
> 
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