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

Paul E. McKenney
2022-04-08T15:50:11Z

On Fri, Apr 08, 2022 at 10:52:21AM -0400, Joel Fernandes wrote:
> On Fri, Apr 8, 2022 at 10:22 AM Paul E. McKenney <paulmck@kernel.org> 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?

I most definitely do. When I first introduced callback offloading, I
made it completely replace softirq callback invocation. There were some
important throughput-oriented workloads that got hit with significant
performance degradation due to this change. Enterprise Java workloads
were the worst hit.

Android does not run these workloads, and I am not aware of ChromeOS
running them, either.

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

That would avoid hurting RHEL and SLES users, so this would be better
than making the change unconditionally. But there are a lot of distros
out there.

I have to ask... Isn't there already a way of specifying a set of kernel
boot parameters that are required for ChromeOS? If so, add rcu_nocbs=0-N
to that list and be happy.

> > 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.

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?

Thanx, Paul