Re: Lazy preemption on arm64

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Dec 17, 2024 at 12:59:31PM +0100, Sebastian Andrzej Siewior wrote:
> 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:
> > > 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.

Yep, I had that in my HACK patch:

  https://lore.kernel.org/linux-rt-users/20241217115931.wjw_HO2V@linutronix.de/T/#m12eece66786a3a207e4e952bdf58570ab75c6a89

... so it sounds like we're on the same page. :)

> > > 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…

I'll try to spin it as a proper patch later this week (and will Cc the
folk here, along with Jinjie, etc); it'll be up to Will and Catalin as
to whether they're happy to pick it up, but given it's small I suspect
that'll be fine.

> > > > 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 :)

Phew, thanks for confirming!

Mark.