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

Peter Zijlstra
2012-09-12T12:53:58Z

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.

Bugger it.. I so detest this patch.