linux-kernel - Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB

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Date:   Thu, 14 Apr 2022 19:19:48 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu <rcu@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for
 RCU_NOCB_CPU=y

On Mon, Apr 11, 2022 at 08:41:09AM -0700, Paul E. McKenney wrote:
[..]
> > > [4]     All CPUs are offloaded at boot, and any CPU can be de-offloaded
> > >         and offloaded at runtime.  This is the same behavior that
> > >         you would currently get with CONFIG_RCU_NOCB_CPU_ALL=n and
> > >         rcu_nocbs=0-N.
> > 
> > Yes, this is the behavior I intend. So then there would not be a need
> > to pass a mask (and I suspect for a large number of users, it
> > simplifies boot params).
> 
> Very good, and from what I can see, this should work for everyone.

Just to clarify, what I am going to do is, if this new option =y, then
rcu_nocbs effectively wont do anything. i.e. All CPUs are offloaded at boot.
Let me know if we are not on the same page about it though. I do feel that is
a sensible choice given =y. If we are on same page, please ignore my comment.

> > > I believe that Steve Rostedt's review would carry weight for ChromeOS,
> > > however, I am suffering a senior moment on the right person for Android.
> > 
> > I think for Android, Kalesh Singh is in the kernel team and Tim Murray
> > is the performance lead. They could appropriately represent their RCU
> > needs.
> 
> Sounds good!  Please collect a Reviewed-by from one or both of them.

Ok.

thanks,

 - Joel