Mark Rutland <mark.rutland@xxxxxxx> writes: > On Wed, Nov 08, 2023 at 04:34:41PM -0800, Ankur Arora wrote: >> Mark Rutland <mark.rutland@xxxxxxx> writes: >> >> > On Tue, Nov 07, 2023 at 01:56:46PM -0800, Ankur Arora wrote: >> >> What's broken: >> >> - ARCH_NO_PREEMPT (See patch-45 "preempt: ARCH_NO_PREEMPT only preempts >> >> lazily") >> >> - Non-x86 architectures. It's trivial to support other archs (only need >> >> to add TIF_NEED_RESCHED_LAZY) but wanted to hold off until I got some >> >> comments on the series. >> >> (From some testing on arm64, didn't find any surprises.) >> > >> > When you say "testing on arm64, didn't find any surprises", I assume you mean >> > with an additional patch adding TIF_NEED_RESCHED_LAZY? >> >> Yeah. And, handling that in the user exit path. >> >> > Note that since arm64 doesn't use the generic entry code, that also requires >> > changes to arm64_preempt_schedule_irq() in arch/arm64/kernel/entry-common.c, to >> > handle TIF_NEED_RESCHED_LAZY. >> >> So, the intent (which got muddied due to this overly large series) >> was to delay handling TIF_NEED_RESCHED_LAZY until we are about to >> return to user. > > Ah, I missed that detail -- thanks for clarifying! > >> I think arm64_preempt_schedule_irq() should only handle TIF_NEED_RESCHED >> and the _TIF_NEED_RESCHED_LAZY should be handled via _TIF_WORK_MASK >> and do_notify_resume(). > > Digging a bit more, I think that should still work. > > One slight clarification: arm64_preempt_schedule_irq() doesn't look at > TIF_NEED_RESCHED today, as it relies on the scheduler IPI calling > preempt_fold_need_resched() to propogate TIF_NEED_RESCHED into > PREEMPT_NEED_RESCHED. That should still work since this series makes > preempt_fold_need_resched() check tif_need_resched(RESCHED_eager). > > I was a bit cnofused because in the generic entry code, > irqentry_exit_cond_resched() explicitly checks for TIF_NEED_RESCHED, and I'm > not sure why it does that rather than relying on the scheduler IPI as above. Yeah I found that confusing as well. I suspect the reason is that not all archs do the folding and we need the explicit check for those that don't. -- ankur
