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

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Message-ID: <aW7D9ghPFdplSBII@localhost.localdomain>
Date: Tue, 20 Jan 2026 00:53:26 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, "Paul E . McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, rcu@...r.kernel.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

Le Mon, Jan 19, 2026 at 06:12:22PM -0500, Joel Fernandes a écrit :
> 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: Frederic Weisbecker <frederic@...nel.org>

Would be nice to have Paul's ack as well, in case we missed something subtle
here.

Also probably for upcoming merge window + 1, note that similar code with
similar removal opportunity resides in rcu_nocb_try_bypass().
And ->nocb_gp_adv_time could then be removed.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs