linux-kernel - Re: is minimum udelay() not respected in preemptible SMP kernel-2.6.23?

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Message-ID: <20071108151003.GS19691@waste.org>
Date:	Thu, 8 Nov 2007 09:10:03 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Andi Kleen <ak@...e.de>, Andrew Morton <akpm@...ux-foundation.org>,
	Marin Mitov <mitov@...p.bas.bg>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: is minimum udelay() not respected in preemptible SMP kernel-2.6.23?

On Thu, Nov 08, 2007 at 01:46:48PM +0200, Avi Kivity wrote:
> Andi Kleen wrote:
> >On Thursday 08 November 2007 01:20, Matt Mackall wrote:
> >  
> >>On Wed, Nov 07, 2007 at 12:30:45PM -0800, Andrew Morton wrote:
> >>    
> >>>Ow.  Yes, from my reading delay_tsc() can return early (or after
> >>>heat-death-of-the-universe) if the TSCs are offset and if preemption
> >>>migrates the calling task between CPUs.
> >>>
> >>>I suppose a lameo fix would be to disable preemption in delay_tsc().
> >>>      
> >>preempt_disable is lousy documentation here. This and other cases
> >>(lots of per_cpu users, IIRC) actually want a migrate_disable() which
> >>is a proper subset. We can simply implement migrate_disable() as
> >>preempt_disable() for now and come back later and implement a proper
> >>migrate_disable() that still allows preemption (and thus avoids the
> >>latency).
> >>    
> >
> >We could actually do this right now. migrate_disable() can be just changing
> >the cpu affinity of the current thread to current cpu and then restoring 
> >it afterwards. That should even work from interrupt context.
> >
> >get_cpu() etc. could be changed to use this then too.
> >
> >  
> 
> What if some other thread calls sched_setaffinity() on the 
> migrate_disable()d cpu?  we'd need to detect this to avoid 
> migrate_enable() stomping on sched_setaffinity()'s work.

Yep, there are a bunch of problems with actually touching the affinity
mask.

-- 
Mathematics is the supreme nostalgia of our time.
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