Le Tue, Apr 21, 2026 at 10:14:09AM -0400, Waiman Long a écrit : 11;rgb:2e2e/3434/3636> On 4/21/26 4:32 AM, Thomas Gleixner wrote: > > On Mon, Apr 20 2026 at 23:03, Waiman Long wrote: > > > To provide nohz_full tick support, there is a set of tick dependency > > > masks that need to be evaluated on every IRQ and context switch. > > s/IRQ/interrupt/ > > > > This is a changelog and not a SMS service. > > > Switching on nohz_full tick support at runtime will be problematic > > > as some of the tick dependency masks may not be properly set causing > > > problem down the road. > > That's useless blurb with zero content. > > > > > Allow nohz_full boot option to be specified without any > > > parameter to force enable nohz_full tick support without any > > > CPU in the tick_nohz_full_mask yet. The context_tracking_key and > > > tick_nohz_full_running flag will be enabled in this case to make > > > tick_nohz_full_enabled() return true. > > I kinda can crystal-ball what you are trying to say here, but that does > > not make it qualified as a proper change log. > > > > > There is still a small performance overhead by force enable nohz_full > > > this way. So it should only be used if there is a chance that some > > > CPUs may become isolated later via the cpuset isolated partition > > > functionality and better CPU isolation closed to nohz_full is desired. > > Why has this key to be enabled on boot if there are no CPUs in the > > isolated mask? > > > > If you want to manage this dynamically at runtime then enable the key > > once CPUs are isolated. Yes, it's more work, but that avoids the "should > > only be used" nonsense and makes this more robust down the road. > > OK, I will try to make it fully dynamic. Of course, it will be more work.
Since the target CPUs will be offline, it should be fine to just enable/disable the static key and masks on runtime. The only issue I see right now is posix CPU timers because the tick dependency is per task/process group. And those tasks could migrate to nohz_full CPUs by careless users (even though that's nonsense) once the target become online. So the per task/process tick dependency must be set up unconditionally. I don't think this should bring much noticeable overhead though. Oh and the other way to go, that is removing TICK_DEP_BIT_POSIX_TIMER and forbid to run posix cpu timers on nohz_full CPUs, would be even more painful to implement so I don't have a better idea. Thanks. -- Frederic Weisbecker SUSE Labs
