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. > (The design is much clearer in Thomas' PoC: > https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/) > > >> - ftrace support for need-resched-lazy is incomplete > > > > What exactly do we need for ftrace here? > > Only support for TIF_NEED_RESCHED_LAZY which should be complete. > That comment was based on a misreading of the code. Cool; thanks! Mark.
