linux-kernel - Re: [patch 1/4] Ignore stolen time in the softlockup watchdog

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Date:	Tue, 24 Apr 2007 14:33:58 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Andi Kleen <ak@...e.de>
Cc:	virtualization@...ts.linux-foundation.org,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Rick Lindsley <ricklind@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	john stultz <johnstul@...ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	virtualization@...ts.osdl.org,
	Chris Lalancette <clalance@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 1/4] Ignore stolen time in the softlockup watchdog

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

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