Re: [PATCH v1 10/11] rcu: Advance callbacks for expedited GP completion in rcu_core()

[Frederic Weisbecker <frederic@kernel.org>]

Le Wed, Jun 24, 2026 at 06:23:52AM -0700, Puranjay Mohan a écrit :
> Even when rcu_pending() triggers rcu_core(), the normal callback
> advancement path through note_gp_changes() -> __note_gp_changes() bails
> out when rdp->gp_seq == rnp->gp_seq (no normal GP change). Since
> expedited GPs do not update rnp->gp_seq, rcu_advance_cbs() is never
> called and callbacks remain stuck in RCU_WAIT_TAIL.
> 
> Add a direct callback advancement block in rcu_core() that checks for GP
> completion via rcu_segcblist_nextgp() combined with
> poll_state_synchronize_rcu_full(). When detected, trylock rnp and call
> rcu_advance_cbs() to move completed callbacks to RCU_DONE_TAIL. Wake the
> GP kthread if rcu_advance_cbs() requests a new grace period.
> 
> Uses trylock to avoid adding contention on rnp->lock. If the lock is
> contended, callbacks will be advanced on the next tick.
> 
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> ---
>  kernel/rcu/tree.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b01d7bf6b57b1..f42e01ef479c4 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2891,6 +2891,23 @@ static __latent_entropy void rcu_core(void)
>  	/* Update RCU state based on any recent quiescent states. */
>  	rcu_check_quiescent_state(rdp);
>  
> +	/* Advance callbacks if an expedited GP has completed. */
> +	if (!rcu_rdp_is_offloaded(rdp) && rcu_segcblist_is_enabled(&rdp->cblist)) {
> +		struct rcu_gp_seq gp_state;
> +
> +		if (rcu_segcblist_nextgp(&rdp->cblist, &gp_state) &&
> +		    poll_state_synchronize_rcu_full(&gp_state)) {
> +			guard(irqsave)();
> +			if (raw_spin_trylock_rcu_node(rnp)) {
> +				bool needwake = rcu_advance_cbs(rnp, rdp);
> +
> +				raw_spin_unlock_rcu_node(rnp);
> +				if (needwake)
> +					rcu_gp_kthread_wake();
> +			}
> +		}
> +	}

Should that go as an improvement to note_gp_changes() instead?

Thanks.

> +
>  	/* No grace period and unregistered callbacks? */
>  	if (!rcu_gp_in_progress() &&
>  	    rcu_segcblist_is_enabled(&rdp->cblist) && !rcu_rdp_is_offloaded(rdp)) {
> -- 
> 2.53.0-Meta
> 

-- 
Frederic Weisbecker
SUSE Labs