On Wed, Nov 08 2023 at 11:13, Peter Zijlstra wrote: > On Wed, Nov 08, 2023 at 02:04:02AM -0800, Ankur Arora wrote: > 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. No. The whole point of LAZY is to keep preempt_schedule(), preempt_schedule_notrace(), irqentry_exit_cond_resched() always enabled. Look at my PoC: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/ The idea is to always enable preempt count and keep _all_ preemption points enabled. For NONE/VOLUNTARY mode let the scheduler set TIF_NEED_RESCHED_LAZY instead of TIF_NEED_RESCHED. In full mode set TIF_NEED_RESCHED. Here is where the regular and the lazy flags are evaluated: Ret2user Ret2kernel PreemptCnt=0 need_resched() NEED_RESCHED Y Y Y Y LAZY_RESCHED Y N N Y The trick is that LAZY is not folded into preempt_count so a 1->0 counter transition won't cause preempt_schedule() to be invoked because the topmost bit (NEED_RESCHED) is set. The scheduler can still decide to set TIF_NEED_RESCHED which will cause an immediate preemption at the next preemption point. This allows to force out a task which loops, e.g. in a massive copy or clear operation, as it did not reach a point where TIF_NEED_RESCHED_LAZY is evaluated after a time which is defined by the scheduler itself. For my PoC I did: 1) Set TIF_NEED_RESCHED_LAZY 2) Set TIF_NEED_RESCHED when the task did not react on TIF_NEED_RESCHED_LAZY within a tick I know that's crude but it just works and obviously requires quite some refinement. So the way how you switch between preemption modes is to select when the scheduler sets TIF_NEED_RESCHED/TIF_NEED_RESCHED_LAZY. No static call switching at all. In full preemption mode it sets always TIF_NEED_RESCHED and otherwise it uses the LAZY bit first, grants some time and then gets out the hammer and sets TIF_NEED_RESCHED when the task did not reach a LAZY preemption point. Which means once the whole thing is in place then the whole PREEMPT_DYNAMIC along with NONE, VOLUNTARY, FULL can go away along with the cond_resched() hackery. So I think this series is backwards. It should add the LAZY muck with a Kconfig switch like I did in my PoC _first_. Once that is working and agreed on, the existing muck can be removed. Thanks, tglx
