On Thu, Apr 14, 2022 at 03:49:16PM -0400, Joel Fernandes wrote: > On Thu, Apr 14, 2022 at 3:42 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > On Thu, Apr 14, 2022 at 07:19:48PM +0000, Joel Fernandes wrote: > > > On Mon, Apr 11, 2022 at 08:41:09AM -0700, Paul E. McKenney wrote: > > > [..] > > > > > > [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. > > > > > > > > > > Yes, this is the behavior I intend. So then there would not be a need > > > > > to pass a mask (and I suspect for a large number of users, it > > > > > simplifies boot params). > > > > > > > > Very good, and from what I can see, this should work for everyone. > > > > > > Just to clarify, what I am going to do is, if this new option =y, then > > > rcu_nocbs effectively wont do anything. i.e. All CPUs are offloaded at boot. > > > Let me know if we are not on the same page about it though. I do feel that is > > > a sensible choice given =y. If we are on same page, please ignore my comment. > > > > I was assuming that the rcu_nocbs=??? for non-empty "???" would override > > the CONFIG_RCU_NOCB_CPU_ALL=y. If you choose not to do that, shouldn't > > you at least issue some sort of diagnostic? After all, the sysadmin > > gave a kernel-boot parameter asking the code to do something and the > > code is choosing not to do that something. > > > > Of course, such a sysadmin might want the CONFIG_RCU_NOCB_CPU_ALL=y > > Kconfig option to affect only the default, that is, when no rcu_nocbs > > kernel boot parameter is specified. This would change the second "[4]" > > in my original table to "[2]". > > > > Thoughts? > > I thought about that. I feel that since we are defaulting the new > config option to =n , it is a conscious choice by the distro to set it > to =y. In such a case, they should be Ok with offloading all CPUs. If > they decide to selectively offload some CPUs in the future, then they > could revisit the config option at that time. > > I feel the kernel config should override the boot parameter behavior. > It is the same effect as a sysadmin passing kernel parameter X > assuming the kernel does something but the CONFIG option might not > even build code corresponding to X. > > I feel to address your concern, we can document in kernel command line > documentation that rcu_nocbs= does not have an effect if > CONFIG_RCU_NOCB_CPU_ALL=y, would that work for you? Not me so much, because I would just set CONFIG_RCU_NOCB_CPU_ALL=n so as to not worry about it. But I am not at all looking forward to complaints about rcu_nocbs not working the way people expect. So let's take some time to think more carefully about this. Thanx, Paul > Thanks, > > - Joel > > > > > > > > > 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. > > > > > > > > > > I think for Android, Kalesh Singh is in the kernel team and Tim Murray > > > > > is the performance lead. They could appropriately represent their RCU > > > > > needs. > > > > > > > > Sounds good! Please collect a Reviewed-by from one or both of them. > > > > > > Ok. > > > > Thanx, Paul
