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

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Message-ID: <3d6e5ca7-5944-4add-90d5-97b672447b5e@paulmck-laptop>
Date: Mon, 19 Jan 2026 16:07:35 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Joel Fernandes <joelagnelf@...dia.com>, linux-kernel@...r.kernel.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

On Tue, Jan 20, 2026 at 12:53:26AM +0100, Frederic Weisbecker wrote:
> 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.

Given that you are good with it, I will take a look.  And test it.  ;-)

> 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.

Further simplification sounds like a good thing!  Just not too simple,
you understand!  ;-)

							Thanx, Paul