Re: [PATCH][retry 1] Add support for Pause Filtering to AMD SVM

Peter Zijlstra
2009-05-11T15:03:40Z

On Mon, 2009-05-11 at 17:51 +0300, Avi Kivity wrote:
> Ingo Molnar wrote:
> > * Avi Kivity <avi@redhat.com> wrote:
> >
> >
> >>> I.e. this is a somewhat poor solution as far as scheduling goes.
> >>> But i'm wondering what the CPU side does. Can REP-NOP really take
> >>> thousands of cycles? If yes, under what circumstances?
> >>>
> >> The guest is running rep-nop in a loop while trying to acquire a
> >> spinlock. The hardware detects this (most likely, repeated
> >> rep-nop with the same rip) and exits. We can program the loop
> >> count; obviously if we're spinning for only a short while it's
> >> better to keep spinning while hoping the lock will be released
> >> soon.
> >>
> >> The idea is to detect that the guest is not making forward
> >> progress and yield. If I could tell the scheduler, you may charge
> >> me a couple of milliseconds, I promise not to sue, that would be
> >> ideal. [...]
> >>
> >
> > Ok, with such a waiver, who could refuse?
> >
> > This really needs a new kernel-internal scheduler API though, which
> > does a lot of fancy things to do:
> >
> > se->vruntime += 1000000;
> >
> > i.e. add 1 msec worth of nanoseconds to the task's timeline. (first
> > remove it from the rbtree, then add it back, and nice-weight it as
> > well)
>
> I suspected it would be as simple as this.

Is that thread guaranteed to run as SCHED_OTHER?