linux-kernel - Re: [PATCH -next v3 2/3] rcu/nocb: Remove dead callback overload handling

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Message-ID: <ec825c7c-01eb-4348-a3a6-e464c51ad22e@paulmck-laptop>
Date: Thu, 22 Jan 2026 21:41:31 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
	rcu@...r.kernel.org, Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>
Subject: Re: [PATCH -next v3 2/3] rcu/nocb: Remove dead callback overload
 handling

On Mon, Jan 19, 2026 at 06:12:22PM -0500, Joel Fernandes wrote:
> During callback overload (exceeding qhimark), the NOCB code attempts
> opportunistic advancement via rcu_advance_cbs_nowake(). Analysis shows
> this entire code path is dead:
> 
> - 30 overload conditions triggered with 300,000 callback flood
> - 0 advancements actually occurred
> - 100% of time blocked because current GP not done
> 
> The overload condition triggers when callbacks are coming in at a high
> rate with GPs not completing as fast. But the advancement requires the
> GP to be complete - a logical contradiction. Even if the GP did complete
> in time, nocb_gp_wait() has to wake up anyway to do the advancement, so
> it is pointless.
> 
> Since the advancement is dead code, the entire overload handling block
> serves no purpose. Remove it entirely.
> 
> Suggested-by: Frederic Weisbecker <frederic@...nel.org>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/rcu/tree_nocb.h | 12 ------------
>  1 file changed, 12 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index f525e4f7985b..64a8ff350f92 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -526,8 +526,6 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
>  				 __releases(rdp->nocb_lock)
>  {
>  	long bypass_len;
> -	unsigned long cur_gp_seq;
> -	unsigned long j;
>  	long lazy_len;
>  	long len;
>  	struct task_struct *t;
> @@ -562,16 +560,6 @@ 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. */
> -		rdp->qlen_last_fqs_check = len;
> -		j = jiffies;
> -		if (j != rdp->nocb_gp_adv_time &&
> -		    rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
> -		    rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
> -			rcu_advance_cbs_nowake(rdp->mynode, rdp);
> -			rdp->nocb_gp_adv_time = j;
> -		}
>  	}
>  
>  	rcu_nocb_unlock(rdp);
> -- 
> 2.34.1
>