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? 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. > 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. > > 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? Mark.
