Re: Lazy preemption on arm64

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-12-17 07:31:51 [+0100], Petr Tesarik wrote:
> V Mon, 16 Dec 2024 19:04:43 +0000
> Mark Rutland <mark.rutland@arm.com> 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@huawei.com/
> > 
> > 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?

Other than that I would be happy to take it then hoping arm64 does the
same.

> 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.

> Petr T

Sebastian