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

Matt Mackall
2007-11-08T15:11:46Z

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.