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

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Message-ID: <aaa33a2c-099f-490a-8054-02dcf4f88ef2@paulmck-laptop>
Date: Tue, 1 Apr 2025 21:24:41 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...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()

On Tue, Apr 01, 2025 at 03:38:21PM +0200, Frederic Weisbecker wrote:
> 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?

The best that I can come up with is hammering with a combination of
expedited grace periods and polled grace periods.

							Thanx, Paul