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

[Joel Fernandes <joel@joelfernandes.org>]

Hi Frederic,

On Thu, Jan 08, 2026 at 06:22:45PM +0100, Frederic Weisbecker wrote:
> Le Thu, Jan 01, 2026 at 11:34:13AM -0500, Joel Fernandes a écrit :
> > To be sure we have no rcog wake ups that were lost, add a warning
> > to cover the case where the rdp is overloaded with callbacks but
> > no wake up was attempted.
> > 
> > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > ---
> >  kernel/rcu/tree.c      | 4 ++++
> >  kernel/rcu/tree.h      | 1 +
> >  kernel/rcu/tree_nocb.h | 6 +++++-
> >  3 files changed, 10 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 293bbd9ac3f4..78c045a5ef03 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3767,6 +3767,10 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
> >  		debug_rcu_head_unqueue(&rdp->barrier_head);
> >  		rcu_barrier_trace(TPS("IRQNQ"), -1, rcu_state.barrier_sequence);
> >  	}
> > +#ifdef CONFIG_RCU_NOCB_CPU
> > +	if (wake_nocb)
> > +		rdp->nocb_gp_wake_attempt = true;
> > +#endif
> 
> entrain only queues a callback if the list is non-empty. And if it's
> non-empty, rdp->nocb_gp_wake_attempt should be true already.

This is true, we don't need to track this wake up. I will replace it with a
WARN.

> >  	rcu_nocb_unlock(rdp);
> >  	if (wake_nocb)
> >  		wake_nocb_gp(rdp, false);
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index 653fb4ba5852..74bd6a2a2f84 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -257,6 +257,7 @@ struct rcu_data {
> >  	unsigned long nocb_gp_loops;	/* # passes through wait code. */
> >  	struct swait_queue_head nocb_gp_wq; /* For nocb kthreads to sleep on. */
> >  	bool nocb_cb_sleep;		/* Is the nocb CB thread asleep? */
> > +	bool nocb_gp_wake_attempt;	/* Was a rcuog wakeup attempted? */
> 
> How about nocb_gp_handling ?

This is a better name indeed, considering that we also track this for
deferred wakeups of the GP thread.

> >  	struct task_struct *nocb_cb_kthread;
> >  	struct list_head nocb_head_rdp; /*
> >  					 * Head of rcu_data list in wakeup chain,
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index daff2756cd90..7e9d465c8ab1 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -546,6 +546,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
> >  	lazy_len = READ_ONCE(rdp->lazy_len);
> >  	if (was_alldone) {
> >  		rdp->qlen_last_fqs_check = len;
> > +		rdp->nocb_gp_wake_attempt = true;
> >  		rcu_nocb_unlock(rdp);
> >  		// Only lazy CBs in bypass list
> >  		if (lazy_len && bypass_len == lazy_len) {
> > @@ -563,7 +564,8 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
> >  
> >  		return;
> >  	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
> > -		/* ... or if many callbacks queued. */
> > +		/* Callback overload condition. */
> > +		WARN_ON_ONCE(!rdp->nocb_gp_wake_attempt);
> >  		rdp->qlen_last_fqs_check = len;
> >  		j = jiffies;
> >  		if (j != rdp->nocb_gp_adv_time &&
> > @@ -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;
  }

  ?

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.

>  		if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
>  			needwake = rdp->nocb_cb_sleep;
> 
> 
> >  			rcu_nocb_unlock_irqrestore(rdp, flags);
> >  			continue; /* No callbacks here, try next. */
> >  		}
> > @@ -1254,6 +1257,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> >  			continue;
> >  		}
> >  		rcu_nocb_try_flush_bypass(rdp, jiffies);
> > +		rdp->nocb_gp_wake_attempt = true;
> 
> Same here, we should expect rdp->nocb_gp_wake_attempt to be already on since
> there are lazy callbacks. That's a good opportunity to test the related assertion
> though.

Good point! I will turn it into a WARN.

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

thanks,

 - Joel