Messages in this thread | | | Date | Thu, 8 Nov 2007 09:10:03 -0600 | From | Matt Mackall <> | Subject | Re: is minimum udelay() not respected in preemptible SMP kernel-2.6.23? |
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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.
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