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

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Date:   Thu, 14 Apr 2022 15:49:16 -0400
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 Thu, Apr 14, 2022 at 3:42 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Thu, Apr 14, 2022 at 07:19:48PM +0000, Joel Fernandes wrote:
> > 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 was assuming that the rcu_nocbs=??? for non-empty "???" would override
> the CONFIG_RCU_NOCB_CPU_ALL=y.  If you choose not to do that, shouldn't
> you at least issue some sort of diagnostic?  After all, the sysadmin
> gave a kernel-boot parameter asking the code to do something and the
> code is choosing not to do that something.
>
> Of course, such a sysadmin might want the CONFIG_RCU_NOCB_CPU_ALL=y
> Kconfig option to affect only the default, that is, when no rcu_nocbs
> kernel boot parameter is specified.  This would change the second "[4]"
> in my original table to "[2]".
>
> Thoughts?

I thought about that. I feel that since we are defaulting the new
config option to =n , it is a conscious choice by the distro to set it
to =y.  In such a case, they should be Ok with offloading all CPUs. If
they decide to selectively offload some CPUs in the future, then they
could revisit the config option at that time.

I feel the kernel config should override the boot parameter behavior.
It is the same effect as a sysadmin passing kernel parameter X
assuming the kernel does something but the CONFIG option might not
even build code corresponding to X.

I feel to address your concern, we can document in kernel command line
documentation that rcu_nocbs= does not have an effect if
CONFIG_RCU_NOCB_CPU_ALL=y, would that work for you?

Thanks,

- Joel

>
> > > > > 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.
>
>                                                         Thanx, Paul