Re: [PATCH tip/core/rcu 11/26] rcu: Exit RCU extended QS on user preemption

Peter Zijlstra
2012-09-12T14:16:19Z

On Wed, 2012-09-12 at 15:54 +0200, Frederic Weisbecker wrote:
> On Wed, Sep 12, 2012 at 02:52:40PM +0200, Peter Zijlstra wrote:
> > On Wed, 2012-09-12 at 14:41 +0200, Peter Zijlstra wrote:
> >
> > > We could of course mandate that all remote wakeups to special nohz cpus
> > > get queued. That would just leave us with RCU and it would simply not
> > > send resched IPIs to extended quiescent CPUs anyway, right?
> > >
> > > So at that point all return to user schedule() calls have nr_running > 1
> > > and the tick is running and RCU is not in extended quiescent state.
> > > Since either we had nr_running > 1 and pre and post state are the same,
> > > or we had nr_running == 1 and we just got a fresh wakeup pushing it to
> > > 2, the wakeup will have executed on our cpu and have re-started the tick
> > > and kicked RCU into active gear again.
> > >
> > > We cannot hit return to user schedule() with nr_running == 0, simply
> > > because in that case there's no userspace to return to, only the idle
> > > thread and that's very much not userspace :-)
> > >
> > > Hmm ?
> >
> > Crap.. this will screw over -rt, since the wakeups batch the IPI can
> > take forever so we had to disable this.
>
> I don't know that part of -rt. Probably we can deal with that later once
> we have some upstream code in place?

Yeah maybe.. its going to be hard though, anyway, I'm going to stare at
wtf drm is doing for a bit :-)