linux-kernel - Re: [PATCH 1/3] rcu: Replace magic number with meaningful constant in rcu_seq_done

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Message-ID: <Z-vsTRydjIAK-4-d@localhost.localdomain>
Date: Tue, 1 Apr 2025 15:38:21 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Joel Fernandes <joelagnelf@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	"rcu@...r.kernel.org" <rcu@...r.kernel.org>
Subject: Re: [PATCH 1/3] rcu: Replace magic number with meaningful constant
 in rcu_seq_done_exact()

Le Mon, Mar 31, 2025 at 02:29:52PM -0700, Paul E. McKenney a écrit :
> The disagreement is a feature, at least up to a point.  That feature
> allows CPUs to go idle for long periods without RCU having to bother
> them or to mess with their per-CPU data (give or take ->gpwrap).  It also
> allows per-rcu_node-leaf locking, which is important on large systems.
> 
> Trying to make precisely globally agreed-on beginnings and ends of
> RCU grace periods will not end well from performance, scalability,
> or real-time-response viewpoints.  ;-)

The distributed disagreement is definetly a feature. The duplicate root
is more debatable.

> But simplifications that don't hurt performance, scalability, and
> real-time-response are of course welcome.

I'm not even sure my proposal is a simplification. Perhaps it is. Another
hope is that it could avoid future accidents.

> Indeed, this probably needs actual performance results showing that
> it is needed.  My guess is that only systems with a single rcu_node
> structure that is both leaf and root would have any chance of noticing.
> And those tend to have few CPUs, so they might not care.

Do you have any idea for a benchmark to test here?

Thanks.

> 							Thanx, Paul