On Mon, Sep 01, 2014 at 01:29:34PM +0200, Peter Zijlstra wrote:
> On Thu, Aug 28, 2014 at 11:26:30AM -0700, Paul E. McKenney wrote:
>
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index b6acb9340192..a1af86099a67 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -2121,16 +2121,23 @@ static void __call_rcu_nocb_enqueue(struct rcu_data
> > *rdp,
> > trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
> > TPS("WakeEmpty"));
> > } else {
> > - rdp->nocb_defer_wakeup = true;
> > + rdp->nocb_defer_wakeup = RCU_NOGP_WAKE;
> > trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
> > TPS("WakeEmptyIsDeferred"));
> > }
> > rdp->qlen_last_fqs_check = 0;
> > } else if (len > rdp->qlen_last_fqs_check + qhimark) {
> > /* ... or if many callbacks queued. */
> > - wake_nocb_leader(rdp, true);
> > + if (!irqs_disabled_flags(flags)) {
> > + wake_nocb_leader(rdp, true);
> > + trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
> > + TPS("WakeOvf"));
> > + } else {
> > + rdp->nocb_defer_wakeup = RCU_NOGP_WAKE_FORCE;
> > + trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu,
> > + TPS("WakeOvfIsDeferred"));
> > + }
> > rdp->qlen_last_fqs_check = LONG_MAX / 2;
> > - trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeOvf"));
> > } else {
> > trace_rcu_nocb_wake(rdp->rsp->name, rdp->cpu, TPS("WakeNot"));
> > }
>
> Is it possible for the RCU_NOCP_WAKE write to overwrite a WAKE_FORCE ?
> If not, why not? (Would make a good comment thereabouts).
Good point.
So RCU_NOGP_WAKE only happens on the empty-to-non-empty transition,
and that RCU_NOGP_WAKE_FORCE only happens once the queue has a large
number of entries (10,000 by default). Because this code runs with irqs
disabled (and must because you can do call_rcu() from interrupt handlers),
RCU_NOGP_WAKE_FORCE could overwrite RCU_NOGP_WAKE, but not vice versa.
However, the same effect can be obtained by having the setting of
RCU_NOGP_WAKE_FORCE race with the call to do_nocb_deferred_wakeup().
However, this would require that the call to do_nocb_deferred_wakeup()
was delayed long enough for 9,999 additional callbacks be registered,
and the next 10,000 callbacks will do another RCU_NOGP_WAKE_FORCE.
In addition, the RCU_NOGP_WAKE_FORCE shortens delays in the rcuo kthreads
rather than waking a rcuo kthread that would otherwise sleep indefinitely.
So I didn't see the point of closing this window.
Thanx, Paul
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