Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y

[Joel Fernandes <joel@joelfernandes.org>]

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.

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.

thanks,

 - Joel



> 							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