linux-kernel - Re: [1/3] rcu: Replace magic number with meaningful constant in rcu_seq_done_exact()

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Message-ID: <8c8932fd34334718e94f979f9bab2fb7.joelagnelf@nvidia.com>
Date: Tue, 01 Apr 2025 12:30:40 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, 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: [1/3] rcu: Replace magic number with meaningful constant in rcu_seq_done_exact() - Patchwork

Hello, Frederic,

On Tue, 1 Apr 2025 16:27:36 GMT, 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.
> 

Aside from the performance concerns:

Sorry if this is silly but could you provide a small hint as to how
unifying the global counter with the node affects QS reporting or hotplug?
It was not immediately obvious to me. Thanks for the help.

Thanks!

 - Joel

> > 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