Hi Paul, On Fri, Apr 8, 2022 at 4:54 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: [...] > > > > > > > > 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] It is always a pleasure to read your well thought out emails ;-) > > [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. Hmm, in other words you are saying that in current code, if only select CPUs are offloaded at boot - then only those can be toggled, but the others are deemed not offload-able? I am happy to leave that quirk/behavior alone as I don't care much right now (for our use cases) for runtime toggling. > [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). > 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. Thanks, - Joel
