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Subject: Re: [patch 1/4] Ignore stolen time in the softlockup watchdog
From: Daniel Walker <dwalker@mvista.com>
To: Andi Kleen <ak@suse.de>
Cc: virtualization@lists.linux-foundation.org,
       Jeremy Fitzhardinge <jeremy@goop.org>,
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Date: Tue, 24 Apr 2007 14:33:58 -0700
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On Tue, 2007-04-24 at 23:20 +0200, Andi Kleen wrote:
> On Tuesday 24 April 2007 22:52:27 Daniel Walker wrote:
> > On Tue, 2007-04-24 at 13:24 -0700, Jeremy Fitzhardinge wrote:
> > 
> > > And sched_clock's use of local_irq_save/restore appears to be absolutely
> > > correct, so I think it must be triggering a bug in either the self-tests
> > > or lockdep itself.
> > 
> > Why does sched_clock need to disable interrupts?
> 
> It's only used in the instable path which is kind of "i already threw up
> my hands" anyways
> 
> I use it because when you transition from stable (TSC) to instable (jiffies)
> the only way to avoid the clock jumping backwards is to remember and update the 
> last value. To avoid races with parallel cpufreq handlers or timer
> interrupts this small section needs to run with interrupts disabled.

Preemption is already disabled with the get_cpu_var() , so it seems like
the timer interrupt is the only worry? I find it confusing that the
access of jiffies_64 isn't protected from interrupts, it's only the
per_cpu data which should already be protected by the
get_cpu_var()/put_cpu_var ..

Daniel