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

Paul E. McKenney
2022-04-08T20:54:50Z

On Fri, Apr 08, 2022 at 02:23:34PM -0400, Joel Fernandes wrote:
> On Fri, Apr 8, 2022 at 2:22 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > On Fri, Apr 8, 2022 at 1:49 PM Paul E. McKenney <paulmck@kernel.org> 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@kernel.org> 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@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.
> > > >
> > > > 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".

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]

[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.

[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.

I am adding Frederic on CC, who will not be shy about correcting any
confusion I be suffering from have with respect to the current code.

Either way, if this is not what you had in mind, what are you suggesting
instead?

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.

Thanx, Paul