On 2024-12-17 11:34:43 [+0000], Mark Rutland wrote: > On Tue, Dec 17, 2024 at 09:50:31AM +0100, Sebastian Andrzej Siewior wrote: > > On 2024-12-17 07:31:51 [+0100], Petr Tesarik wrote: > > > V Mon, 16 Dec 2024 19:04:43 +0000 > > > Mark Rutland <mark.rutland@xxxxxxx> napsáno: > > > > > > > On Mon, Dec 16, 2024 at 07:04:51PM +0100, Petr Tesarik wrote: > > > > > Hi all, > > > > > > > > > > what is the plan for implementing PREEMPT_LAZY on arm64? > > > > > > > > > > There used to be RT patch series which enabled lazy preemption on > > > > > arm64, but this architecture was "sacrificed" in v6.6-rc6-rt10, as > > > > > collateral damage of switching to PREEMPT_AUTO. > > > > > > > > > > IIUC lazy preemption is currently implemented only for architectures > > > > > with CONFIG_GENERIC_ENTRY, but there is no inherent dependency on it. > > > > > So, is the plan to convert arm64 to GENERIC_ENTRY (and then get > > > > > PREEMPT_LAZY for free), or is somebody working on CONFIG_PREEMPT_LAZY > > > > > for arm64 without that conversion? > > > > > > > > I don't think there's an agreed upon plan either way. > > > > > > > > Jinjie Ruan has been looking to move arm64 over to GENERIC_ENTRY: > > > > > > > > https://lore.kernel.org/all/20241206101744.4161990-1-ruanjinjie@xxxxxxxxxx/ > > > > > > > > AFAICT, the only bits that we get "for free" from GENERIC_ENTRY would be > > > > the logic in raw_irqentry_exit_cond_resched() and > > > > exit_to_user_mode_loop(), and all we'd need to enable this on arm64 > > > > as-is would be as below. > > > > > > @bigeasy: Would it be OK for you to add the below patch to the next > > > 6.13 RT patches? > > > > This bits below are actually the same ones I made last week. I stopped > > there because it was late and I didn't find GENERIC_ENTRY nor a > > TIF_NEED_RESCHED check in arm64 so I paused. Where is this? > > Currently arm64 doesn't use GENERIC_ENTRY; people are working on that > (see the link above), but it's likely to take a short while. IIUC > there's no strict dependency on GENERIC_ENTRY here, unless I'm missing > something? No, not really, that is perfect. > For TIF_NEED_RESCHED, arm64 relies upon the core code to call > set_preempt_need_resched() (e.g. via preempt_fold_need_resched()) to > fold that into thread_info::preempt::need_resched. That's checked by > arm64_preempt_schedule_irq(), which reads thread_info::preempt_count, > which is unioned with thread_info::preempt::{count,need_resched} such > that the two fields can be checked together. All sounds fine. Now, if that bit is set, we need schedule() before returning to userland. I didn't it initially but now I did: diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c index b260ddc4d3e9a..2e2f13ce076da 100644 --- a/arch/arm64/kernel/entry-common.c +++ b/arch/arm64/kernel/entry-common.c @@ -132,7 +132,7 @@ static void do_notify_resume(struct pt_regs *regs, unsigned long thread_flags) do { local_irq_enable(); - if (thread_flags & _TIF_NEED_RESCHED) + if (thread_flags & _TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY) schedule(); if (thread_flags & _TIF_UPROBE) With that piece we should be fine. > > Other than that I would be happy to take it then hoping arm64 does the > > same. > > If PREEMPT_LAZY is something that people need urgently then I can go > turn the hack into a proepr patch and see if we can queue that ahead of > the larger rework for GENERIC_ENTRY. I would appreciate it. However if there is reason to delay it I could hold to it for some time… > > > Mark tagged it with "HACK", but to me it actually looks just as good as > > > the good old (pre-PREEMPT_AUTO) arm64 patch. ;-) > > > > The old lazy-preempt had also tweaks in should_resched() and > > __preempt_count_dec_and_test(). So it is slightly different. > > Hmm... what needed to change there? > > Currently we're relying on the union trick to check both > thread_info::preempt::{count,need_resched}, where the latter should have > TIF_NEED_RESCHED folded in (but not TIF_NEED_RESCHED_LAZY), which IIUC > is sufficient? The old lazy-preempt dates back to around v3.0-RT+. The logic back then was slightly different and had also a counter (similar to the counter used by preempt_disable()) so we had to ensure preempt_enable() does not schedule if the lazy-counter > 0 and the caller was not a RT task. With the improvements over time and the current design a lot of the old cruft simply removed. So nothing to worry :) > Mark. Sebastian
