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

[Joel Fernandes]

On Fri, Apr 8, 2022 at 2:22 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Apr 8, 2022 at 1:49 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >
> > On Fri, Apr 08, 2022 at 01:20:02PM -0400, Joel Fernandes wrote:
> > > On Fri, Apr 8, 2022 at 11:50 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > > >
> > > > 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@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?
> > > >
> > > > 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.
> > >
> > > Thanks a lot for mentioning this, I was not aware and will make note
> > > of it :-). I wonder if the scheduler had something to do with the
> > > degradation.
> >
> > It is all too easy to blame the scheduler and all too easy to forget
> > that the scheduler has a hard job.  ;-)
> >
> > And in this case, the scheduler was just doing what it was told.
>
> No was just saying the scheduler has to do more work with NOCB because
> of the extra threads, so that likely degrades the workloads (context
> switch, wake ups, etc).
>
> > > > > > 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".

thanks,

- Joel