linux-kernel - Re: NMI watchdog + NOHZ question

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Message-Id: <20090624.000811.118222188.davem@davemloft.net>
Date:	Wed, 24 Jun 2009 00:08:11 -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 09:03:15 +0200

>> I'm not exactly sure what to do about this.
> 
> Ack the timer interrupt earlier (and also give it a high priority?)

It has a higher priority, but all interrupts get re-enabled right
before we process software interrupts.  So the flood of qla2xxx
interrupts can come in before we can run the timer softirq and
thus schedule the next timer interrupt.

> That could be still problematic if you have non nestabled irq stacks
> (haven't checked if sparc has that or not), 
> potentially you might need to run the softirq on the process stack.

IRQ stacks on sparc64 work identically to how they do on x86.

I have some more theories about this, in that I always see the
NMI watchdog message with a PC right in the section of CPU idle
where NOHZ is enabled.

On these cpus there is no support yielding, so on them I just
touch the NMI watchdog in the loop waiting for need_resched()
to become true.

But if we get the qla2xxx interrupt storm during that loop, it's
pretty easy to not touch the NMI watchdog in time.
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