Re: [Bug #12208] uml is very slow on 2.6.28 host

Mike Galbraith
2008-12-16T03:25:27Z

On Tue, 2008-12-16 at 01:49 +0100, Miklos Szeredi wrote:
> On Sat, 13 Dec 2008, Rafael J. Wysocki wrote:
> > Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=12208
> > Subject : uml is very slow on 2.6.28 host
> > Submitter : Miklos Szeredi <miklos-sUDqSbJrdHQHWmgEVkV9KA@public.gmane.org>
> > Date : 2008-12-12 9:35 (2 days old)
> > References : http://marc.info/?l=linux-kernel&m=122907463518593&w=4
>
> I did a bisection, and this is the commit which is responsible:
>
> commit 464b75273f64be7c81fee975bd6ca9593df3427b
> Author: Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org>
> Date: Fri Oct 24 11:06:15 2008 +0200
>
> sched: re-instate vruntime based wakeup preemption
>
> The advantage is that vruntime based wakeup preemption has a better
> conceptual model. Here wakeup_gran = 0 means: preempt when 'fair'.
> Therefore wakeup_gran is the granularity of unfairness we allow in order
> to make progress.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org>
> Acked-by: Mike Galbraith <efault-Mmb7MZpHnFY@public.gmane.org>
> Signed-off-by: Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>

If that commit is responsible, then it should also be very slow in pre
28 kernels, where the same exists. Hm, there's another possibility.
Can you try echo NO_LAST_BUDDY > /sys/kernel/debug/sched_features?

-Mike