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

[Joel Fernandes]

Hi Paul,

On Fri, Apr 8, 2022 at 4:54 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
[...]
> > > > > > > > And is it really all -that- hard to specify an additional boot parameter
> > > > > > > > across ChromeOS devices?  Android seems to manage it.  ;-)
> > > > > > >
> > > > > > > That's not the hard part I think. The hard part is to make sure a
> > > > > > > future Linux user who is not an RCU expert does not forget to turn it
> > > > > > > on. ChromeOS is not the only OS that I've seen someone forget to do it
> > > > > > > ;-D. AFAIR, there were Android devices too in the past where I saw
> > > > > > > this forgotten. I don't think we should rely on the users doing the
> > > > > > > right thing (as much as possible).
> > > > > > >
> > > > > > > The single kernel binary point makes sense but in this case, I think
> > > > > > > the bigger question that I'd have is what is the default behavior and
> > > > > > > what do *most* users of RCU want. So we can keep sane defaults for the
> > > > > > > majority and reduce human errors related to configuration.
> > > > > >
> > > > > > If both the ChromeOS and Android guys need it, I could reinstate the
> > > > > > old RCU_NOCB_CPU_ALL Kconfig option.  This was removed due to complaints
> > > > > > about RCU Kconfig complexity, but I believe that Reviewed-by from ChromeOS
> > > > > > and Android movers and shakers would overcome lingering objections.
> > > > > >
> > > > > > Would that help?
> > > > >
> > > > > Yes, I think I would love for such a change. I am planning to add a
> > > > > test to ChromeOS to check whether config options were correctly set
> > > > > up. So I can test for both the RCU_NOCB_CPU options.
> > > >
> > > > Very good!
> > > >
> > > > Do you love such a change enough to create the patch and to collect
> > > > convincing Reviewed-by tags?
> > >
> > > Yes sure, just so I understand - basically I have to make the code in
> > > my patch run when RCU_NOCB_CPU_ALL option is passed (and keep the
> > > option default disabled), but otherwise default to the current
> > > behavior, right?
> >
> > Sorry rephrasing, "make the code in my patch run when the new
> > CONFIG_RCU_NOCB_CPU_ALL is enabled".
>
> Here is what I believe you are proposing:
>
>
>                                 ---     rcu_nocbs       rcu_nocbs=???
>
> CONFIG_RCU_NOCB_CPU_ALL=n       [1]     [2]             [3]
>
> CONFIG_RCU_NOCB_CPU_ALL=y       [4]     [4]             [3]

It is always a pleasure to read your well thought out emails ;-)

>
> [1]     No CPUs are offloaded at boot.  CPUs cannot be offloaded at
>         runtime.
>
> [2]     No CPUs are offloaded at boot, but any CPU can be offloaded
>         (and later de-offloaded) at runtime.
>
> [3]     The set of CPUs that are offloaded at boot are specified by the
>         mask, represented above with "???".  The CPUs that are offloaded
>         at boot can be de-offloaded and offloaded at runtime.  The CPUs
>         not offloaded at boot cannot be offloaded at runtime.

Hmm, in other words you are saying that in current code, if only
select CPUs are offloaded at boot - then only those can be toggled,
but the others are deemed not offload-able? I am happy to leave that
quirk/behavior alone as I don't care much right now (for our use
cases) for runtime toggling.

> [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).

> 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.

Thanks,

 - Joel