Re: [PATCH -next 4/8] rcu/nocb: Add warning if no rcuog wake up attempt happened during overload

[Frederic Weisbecker <frederic@kernel.org>]

Le Thu, Jan 08, 2026 at 10:49:30PM -0500, Joel Fernandes a écrit :
> > > @@ -688,6 +690,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> > >  		     bypass_ncbs > 2 * qhimark)) {
> > >  			flush_bypass = true;
> > >  		} else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
> > > +			rdp->nocb_gp_wake_attempt = false;
> > 
> > This is when nocb_cb_wait() is done with callbacks but nocb_gp_wait() is done
> > with them sooner, when the grace period is done for all pending callbacks.
> > 
> > Something like this would perhaps be more accurate:
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index e6cd56603cad..52010cbeaa76 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -746,6 +746,8 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> >  			needwait_gp = true;
> >  			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> >  					    TPS("NeedWaitGP"));
> > +		} else if (!rcu_cblist_n_cbs(&rdp->nocb_bypass)) {
> > +			rdp->nocb_gp_wake_attempt = false;
> >  		}
> 
> Hmm, I am trying to understand why this suggestion is better than what I
> already have. It is one extra line and adds another conditional.
> 
> Also shouldn't it be:
> 
>   } else if (!rcu_cblist_n_cbs(&rdp->nocb_bypass) &&
>              rcu_segcblist_empty(&rdp->cblist)) {
>       rdp->nocb_gp_wake_attempt = false;
>   }
> 
>   ?

This else already means that rcu_segcblist_nextgp() returned false because there
is no pending callbacks. rcu_segcblist_empty() is different because it also test
done callbacks.

> 
> My goal was to mark wake_attempt as false when ALL callbacks on the rdp were
> drained. IOW, the GP thread is done with the rdp.

So nocb_gp_wait (the rcuog kthreads) handle the pending callbacks, advancing
them throughout grace periods until they are moved to the done state.

But once they are moved as done, the callbacks are the responsibility of
nocb_cb_wait() (the rcuo kthreads) and nocb_gp_wait() stops paying attention
to that rdp if there are no more pending callbacks.

So with your initial patch, you still have rdp->nocb_gp_wake_attempt == true
even when there are only done callbacks. But without an appropriate wake-up
after subsequent enqueue, nocb_gp_wait() may ignore new callbacks, event though
rdp->nocb_gp_wake_attempt == true suggests otherwise.

> Btw, I have more patches coming to simplify nocb_gp_wait()... it is quite long
> :)

Cool :)

Thanks.

-- 
Frederic Weisbecker
SUSE Labs