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

Frederic Weisbecker
2012-09-12T13:55:09Z

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?

>
> Bugger it.. I so detest this patch.