linux-kernel - Re: 'global' rq->clock

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Date:	Sun, 04 May 2008 12:22:47 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Miller <davem@...emloft.net>, a.p.zijlstra@...llo.nl,
	efault@....de, elendil@...net.nl, parag.warudkar@...il.com,
	linux-kernel@...r.kernel.org, guichaz@...oo.fr, andi@...stfloor.org
Subject: Re: 'global' rq->clock


> on nohz we still keep jiffies uptodate - despite there not being an 
> explicit 'keep jiffies uptodate' tick interrupt anymore. So on every 
> irq_enter() we roll jiffies forward - if needed - and thus emulate 
> jiffies behavior to drivers and core kernel code, etc.
> 
> if an IPI on Power does not do an irq_enter() then you might miss out on 
> updated jiffies. That might not matter for most jiffies, but you might 
> also miss out on the 'touch the softlockup watchdog because we just woke 
> from idle' action. This is what triggered the false positive warnings on 
> Sparc64.
> 
> the same bug existed on x86 too: that too does a few IPIs without 
> irq_enter/irq_exit. We now removed the softlockup dependency so it 
> should not be required to do an irq_enter()/exit anymore - unless the 
> code that the IPI uses accesses jiffies. (but that would be unusual)

Allright, so it looks like we don't have a problem today, though I'll be
careful if some of our CPUs come up with more fancy ways to do IPIs.
Currently they are just normal interrupts, so we do irq_enter/exit and
we process softirqs on the way out.

Ben.


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