Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes: > On Wed, Nov 08, 2023 at 02:04:02AM -0800, Ankur Arora wrote: > >> >> Revert "riscv: support PREEMPT_DYNAMIC with static keys" >> >> Revert "livepatch,sched: Add livepatch task switching to >> >> cond_resched()" >> >> Revert "arm64: Support PREEMPT_DYNAMIC" >> >> Revert "sched/preempt: Add PREEMPT_DYNAMIC using static keys" >> >> What's the best way to handle these? With the lazy bit, cond_resched() >> and might_resched() are gone. So we don't need all of the static >> key inftrastructure for toggling etc. >> >> The part of preempt_dynamic that makes sense to me is the one that >> switches dynamically between none/voluntary/full. Here it would need >> to be wired onto controls of the lazy bit. >> (Right now the preemption policy is controlled by sched_feat in >> patches 43, and 44 but sched/preempt is a much better interface.) > > I'm not understanding, those should stay obviously. > > The current preempt_dynamic stuff has 5 toggles: > > /* > * SC:cond_resched > * SC:might_resched > * SC:preempt_schedule > * SC:preempt_schedule_notrace > * SC:irqentry_exit_cond_resched > * > * > * NONE: > * cond_resched <- __cond_resched > * might_resched <- RET0 > * preempt_schedule <- NOP > * preempt_schedule_notrace <- NOP > * irqentry_exit_cond_resched <- NOP > * > * VOLUNTARY: > * cond_resched <- __cond_resched > * might_resched <- __cond_resched > * preempt_schedule <- NOP > * preempt_schedule_notrace <- NOP > * irqentry_exit_cond_resched <- NOP > * > * FULL: > * cond_resched <- RET0 > * might_resched <- RET0 > * preempt_schedule <- preempt_schedule > * preempt_schedule_notrace <- preempt_schedule_notrace > * irqentry_exit_cond_resched <- irqentry_exit_cond_resched > */ > > If you kill voluntary as we know it today, you can remove cond_resched > and might_resched, but the remaining 3 are still needed to switch > between NONE and FULL. Ah now I see what you are saying. Quick thought: even if we were running under NONE, eventually you'll want to forcibly preempt out a CPU hog. So we will need to have at least this one enabled always: > * irqentry_exit_cond_resched <- irqentry_exit_cond_resched These two, it might make sense to toggle them based on model. > * preempt_schedule <- preempt_schedule > * preempt_schedule_notrace <- preempt_schedule_notrace Anyway let me think about this more and respond tomorrow. For now, time for bed. Thanks for clarifying btw. Ankur > Additionally, you'll get one new state to enable/disable the LAZY stuff. > Neither NONE nor FULL want the LAZY thing on. > > You'll then end up with something like: > > /* > * SK:preempt_lazy > * SC:preempt_schedule > * SC:preempt_schedule_notrace > * SC:irqentry_exit_cond_resched > * > * > * NONE: > * preempt_lazy <- OFF > * preempt_schedule <- NOP > * preempt_schedule_notrace <- NOP > * irqentry_exit_cond_resched <- NOP > * > * VOLUNTARY: > * preempt_lazy <- ON > * preempt_schedule <- preempt_schedule > * preempt_schedule_notrace <- preempt_schedule_notrace > * irqentry_exit_cond_resched <- irqentry_exit_cond_resched > * > * FULL: > * preempt_lazy <- OFF > * preempt_schedule <- preempt_schedule > * preempt_schedule_notrace <- preempt_schedule_notrace > * irqentry_exit_cond_resched <- irqentry_exit_cond_resched > */ > > For the architectures that do not have static_call but instead use > static_key for everything, the SC's are obviously static_key based > wrappers around the function calls -- like now.
