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

Paul E. McKenney
2012-09-12T15:23:02Z

On Wed, Sep 12, 2012 at 02:41:36PM +0200, Peter Zijlstra wrote:
> On Wed, 2012-09-12 at 14:06 +0200, Frederic Weisbecker wrote:
> >
> > 1) This can happen if something calls set_need_resched() while no other task is
> > on the runqueue.
>
> People really shouldn't be doing that... I think I know why RCU does
> this, but yuck. I also think RCU can avoid doing this, but its a toss up
> if that's worth the trouble.

It used to do this whenever it had to force quiescent states on a given
CPU more than one time in a given grace period. I have removed this
(on your advice), and the only remaining use is during RCU CPU stall
warnings, where a given CPU has refused to pass through a quiescent
state for more than a minute.

I could remove that one as well, if it would help. Though it would
be nice to have -some- way to kick the CPU in that case. Thoughts?

> > 2) Remote wake up done but we haven't yet received the schedule IPI.
> >
> > 3) Non IPI remote wakeup you're referring above, I'm not sure
> > what you mean though.
>
> Well there's two ways of doing remote wakeups, one is doing the wakeup
> from the waking cpu and sending an IPI over to reschedule iff you need
> wakeup-preemption, the other is queueing the task remotely and sending
> an IPI to do the wakeup on the remote cpu.
>
> The former has the problem, the latter not.
>
> See ttwu_queue().
>
> 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?

Indeed it never has done this, at least not in the absence of bugs.

Thanx, Paul

> 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 ?
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