Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested

Paul E. McKenney
2018-07-19T03:12:03Z

On Thu, Jul 19, 2018 at 02:32:06AM +0200, Frederic Weisbecker wrote:
> On Wed, Jul 11, 2018 at 06:03:42PM +0100, David Woodhouse wrote:
> > On Wed, 2018-07-11 at 09:49 -0700, Paul E. McKenney wrote:
> > > And here is an updated v4.15 patch with Marius's Reported-by and David's
> > > fix to my lost exclamation point.
> >
> > Thanks. Are you sending the original version of that to Linus? It'd be
> > useful to have the commit ID so that we can watch for it landing, and
> > chase this one up to Greg.
> >
> > As discussed on IRC, this patch reduces synchronize_sched() latency for
> > us from ~4600s to ~160ms, which is nice.
> >
> > However, it isn't going to be sufficient in the NO_HZ_FULL case. For
> > that you want a patch like the one below, which happily reduces the
> > latency in our (!NO_HZ_FULL) case still further to ~40ms.
>
> That is interesting. As I replied to Paul, we are already calling
> rcu_user_enter/exit() on guest_enter/exit_irqsoff(). So I'm wondering why
> you're seeing such an optimization by repeating those calls.
>
> Perhaps the rcu_user_* somehow aren't actually called from
> __context_tracking_enter()...? Some bug in context tracking?
> Otherwise it's a curious side effect.

David is working with v4.15. Is this maybe something that has changed
since then?

Thanx, Paul