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. > 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
