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
