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

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Message-ID: <816a14e0-c06e-496a-9c84-512c76d98157@paulmck-laptop>
Date: Fri, 23 Jan 2026 13:27:46 -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 Fri, Jan 23, 2026 at 02:36:37PM -0500, Joel Fernandes wrote:
> On 1/23/2026 11:49 AM, Paul E. McKenney wrote:
> > We could have one CPU flooding and the rest idle, and many other
> > combinations.  And, if I recall correctly, polling can burn extra CPU
> > and cause extra wakeups even when the system is fully idle.  Or has
> > that changed?
> 
> In my experience working on lazy RCU, if you have such a kind of overload on
> any CPU, then you're usually not saving any power anyway. The system has to
> be really quiet and idle with a low stream of callbacks for you to save
> power. Further, when the callback length increases too much, we don't turn
> on lazy RCU anyway because the idea is that we are overloaded and the
> system is busy - so we already have such assumptions baked in. I think a
> similar argument could apply here for dynamically enabling polling mode only
> when overloaded.

The concern is detecting overload quickly.  Any unnecessary gaps in
invoking RCU callbacks cannot be made up.  That time is gone.
And the polling does sleeps...

> I was coming more from the point of view of improving grace period performance
> when we do have an overload, potentially resolving the overloaded situation
> faster than usual. We would dynamically trigger polling based on such
> circumstances.
> 
> That said, I confess I don't have extensive experience with polling mode beyond
> testing. I believe we should add more rcutorture test cases for this. I'm
> considering adding a new config that enables polling for NOCB - this testing is
> what revealed the potential for grace period performance improvement with NOCB
> to me.

The main purpose of polling was to make call_rcu() avoid at least some
of its slowpaths.  If we are getting some other benefit out of it, is
polling the best way to achieve that benefit?

							Thanx, Paul