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In-Reply-To: <20220414213156.GZ4285@paulmck-ThinkPad-P17-Gen-1>
From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Thu, 14 Apr 2022 17:38:08 -0400
Message-ID: <CAEXW_YT0CDp7kqKVeqGRT8YfXiCYuL0ZgRwz05MQrmns2Yp+2Q@mail.gmail.com>
Subject: Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y
To:     "Paul E. McKenney" <paulmck@kernel.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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, Apr 14, 2022 at 5:31 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Thu, Apr 14, 2022 at 09:14:05PM +0000, Joel Fernandes wrote:
> > On Thu, Apr 14, 2022 at 02:09:33PM -0700, Paul E. McKenney wrote:
> > > 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.
> >
> > That's a fair concern. But we are defaulting it to 'n' so I think if it is
> > unconsciously enabled without someone reading documentation, then that's a
> > slightly different issue.
>
> Suppose that one group decides to change to CONFIG_RCU_NOCB_CPU_ALL=y,
> and some other group on some other continent happens to be using the
> rcu_nocbs boot parameter (having read the documentation).  And suppose
> that the level of communication between the two groups is typical,
> that is to say, nonexistent.
>
> Sure, we can argue that groups should communicate, but our making that
> argument won't necessarily prevent the group using rcu_nocbs from taking
> us to task in the course of their debugging effort.
>
> > On the other hand, I can also make it such that if rcu_nocbs= is passed, then
> > the CONFIG does not take effect. That's quite a bit weird/quirky IMHO.
>
> Not at all.  We can simply say that CONFIG_RCU_NOCB_CPU_ALL controls
> only the default situation, that is, when rcu_nocbs is not specified.

Then it should be called: CONFIG_RCU_NOCB_CPU_DEFAULT_ALL , or
something. Otherwise I can tell you that I will be the first one to be
confused by menuconfig unless I also read the code :-D

thanks,

- Joel