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? 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
