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

[Joel Fernandes]

On Fri, Apr 8, 2022 at 10:22 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Thu, Apr 07, 2022 at 09:07:33PM +0000, Joel Fernandes wrote:
> > On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
> > which ends up not offloading any CPU. This patch removes yet another
> > dependency from the bootloader having to know about RCU, about how many
> > CPUs the system has, and about how to provide the mask. Basically, I
> > think we should stop pretending that the user knows what they are doing :).
> > In other words, if NO_CB_CPU is enabled, lets make use of it.
> >
> > My goal is to make RCU as zero-config as possible with sane defaults. If
> > user wants to provide rcu_nocbs= or nohz_full= options, then those will
> > take precedence and this patch will have no effect.
> >
> > I tested providing rcu_nocbs= option, ensuring that is preferred over this.
>
> Unless something has changed, this would change behavior relied upon
> the enterprise distros.  Last I checked, they want to supply a single
> binary, as evidenced by the recent CONFIG_PREEMPT_DYNAMIC Kconfig option,
> and they also want the default to be non-offloaded.  That is, given a
> kernel built with CONFIG_RCU_NOCB_CPU=y and without either a nohz_full
> or a nocbs_cpu boot parameter, all of the CPUs must be non-offloaded.

Just curious, do you have information (like data, experiment results)
on why they want default non-offloaded? Or maybe they haven't tried
the recent work done in NOCB code?

Another option I think is to make it enforce NOCB if NR_CPUS <= 32 if
that makes sense.

> So for me to push this to mainline, I need an ack from someone from each
> of the enterprise distros, and each of those someones needs to understand
> the single-binary strategy used by the corresponding distro.

Ok.

> 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.

thanks,

-Joel