linux-kernel - Re: regression from softlockup fix

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Date:	Mon, 19 Nov 2007 09:15:38 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	gregkh@...e.de, Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: regression from softlockup fix

Ingo Molnar wrote:
> * David Miller <davem@...emloft.net> wrote:
>
>   
>> I suspect that what is happening is that the NOHZ period is longer 
>> than the softlockup timeout (10 seconds) and we get an interrupt 
>> before the watchdog thread gets onto the cpu.
>>     
>
> indeed! Does the patch below do the trick?
>
> 	Ingo
>
> --------------->
> Subject: softlockup: do the wakeup from a hrtimer
> From: Ingo Molnar <mingo@...e.hu>
>
> David Miller reported soft lockup false-positives that trigger
> on NOHZ due to CPUs idling for more than 10 seconds.
>
> The solution is to drive the wakeup of the watchdog threads
> not from the timer tick (which has no guaranteed frequency),
> but from the watchdog tasks themselves.
>   

I thought the timer code kicked the watchdog after waking up after a
long sleep anyway?  At one point I was looking into a mechanism to
temporarily disable the watchdog during a wait for a timer event, but it
got complex - and I thought - unnecessary.

Specifically this in kernel/time/timekeeping.c:

		/*
		 * When we are idle and the tick is stopped, we have to touch
		 * the watchdog as we might not schedule for a really long
		 * time. This happens on complete idle SMP systems while
		 * waiting on the login prompt. We also increment the "start of
		 * idle" jiffy stamp so the idle accounting adjustment we do
		 * when we go busy again does not account too much ticks.
		 */
		if (ts->tick_stopped) {
			touch_softlockup_watchdog();
			ts->idle_jiffies++;
		}

Or does this happen on the sleep path?  If so, wouldn't the right fix to
be this on the wakeup path?

    J
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