From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
rcu <rcu@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y
Date: Thu, 14 Apr 2022 14:09:33 -0700 [thread overview]
Message-ID: <20220414210933.GW4285@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <CAEXW_YS5b_51tqmdf27QqQHqsgJKbTys1V3h+Bek3XN4FjBmbw@mail.gmail.com>
On Thu, Apr 14, 2022 at 03:49:16PM -0400, Joel Fernandes wrote:
> On Thu, Apr 14, 2022 at 3:42 PM Paul E. McKenney <paulmck@kernel.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?
Not me so much, because I would just set CONFIG_RCU_NOCB_CPU_ALL=n so
as to not worry about it.
But I am not at all looking forward to complaints about rcu_nocbs not
working the way people expect. So let's take some time to think more
carefully about this.
Thanx, Paul
> 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
next prev parent reply other threads:[~2022-04-14 21:09 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-07 21:07 [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y Joel Fernandes
2022-04-08 14:22 ` Paul E. McKenney
2022-04-08 14:52 ` Joel Fernandes
2022-04-08 15:50 ` Paul E. McKenney
2022-04-08 17:20 ` Joel Fernandes
2022-04-08 17:49 ` Paul E. McKenney
2022-04-08 18:22 ` Joel Fernandes
2022-04-08 18:23 ` Joel Fernandes
2022-04-08 20:54 ` Paul E. McKenney
2022-04-08 21:46 ` Uladzislau Rezki
2022-04-11 14:08 ` Paul E. McKenney
2022-04-11 15:20 ` Uladzislau Rezki
2022-04-11 15:17 ` Joel Fernandes
2022-04-11 15:41 ` Paul E. McKenney
2022-04-14 19:19 ` Joel Fernandes
2022-04-14 19:42 ` Paul E. McKenney
2022-04-14 19:49 ` Joel Fernandes
2022-04-14 19:51 ` Joel Fernandes
2022-04-14 21:10 ` Paul E. McKenney
2022-04-14 21:09 ` Paul E. McKenney [this message]
2022-04-14 21:14 ` Joel Fernandes
2022-04-14 21:31 ` Paul E. McKenney
2022-04-14 21:38 ` Joel Fernandes
2022-04-14 22:37 ` Paul E. McKenney
2022-04-20 20:36 ` Steven Rostedt
2022-04-11 13:49 ` Uladzislau Rezki
2022-04-11 15:17 ` Joel Fernandes
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