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On 9/3/26 3:23 PM, Michal Suchánek wrote: > On Thu, Feb 26, 2026 at 02:24:50PM +0100, Christophe Leroy (CS GROUP) wrote: >> Hi Christian, >> >> Le 26/02/2026 à 06:37, Christian Zigotzky a écrit : >>> On 25/02/26 08:16, Christophe Leroy (CS GROUP) wrote: >>>> +Peter >>>> >>>> Hi Christian, >>>> >>>> Le 25/02/2026 à 06:54, Christian Zigotzky a écrit : >>>>> Hello, >>>>> >>>>> We were previously able to disable PREEMPT in the kernel >>>>> configuration, but the latest kernels now enable it by default >>>>> and it is no longer possible to disable it. >>>>> >>>>> Is it possible to let us decide whether to activate PREEMPT or not? >>>> >>>> Can you give more details on why you want to be able to deactivate >>>> PREEMPT ? >>>> >>>> Read https://eur01.safelinks.protection.outlook.com/? url=https%3A%2F%2Fgithub.com%2Ftorvalds%2Flinux%2Fcommit%2F7dadeaa6e851e&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C7006c8b4b98c4315a7e508de74f9388a%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639076810956939768%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=6xLB%2FOVSkmgZRamaBM9zaxm%2FUTk9ssf4409QJdiXkDg%3D&reserved=0 >>> I use PREEMPT_NONE kernels on my Linux PPC servers. (Without desktop) >> >> You are doing that today because it is possible. Tomorrow it will not be >> possible anymore, for the reasons explained by Peter. >> >> My question was not to know what you are doing but why, and why you think >> you still need PREEMPT_NONE and why the modern PREEMPT_LAZY is not good >> enough for you. > > Hello, > > I tried to compile a kernel with preemption for pSeries, and while > PREEMPT_VOLUNTARY works for me PREEMPT_LAZY crashes. > > The kernel crashing is certainly a reason why a preemtion model would > not be good enough for me. > Is it the same issue that you said the bug which was discussed internally? i.e around kvmhv_run_single_vcpu ? or is it different one? > This is not the latest kernel so it might have been fixed in the > meantime for the platform in question. Will need to check that. > If possible run against current upstream and share the results. > Nonethless, there are quite a few platforms. Originally no preemption > was the only option, and that's the reason why many people run that. > It's the conservative, known working option. And that's the reason a lot > of platfrom code does not get tested with more aggressive preemtion > models, and never gets fixed to work with them. > Full preemption has been there for many years!. Lazy is not that aggressive compared to that. > Simply disabling the no preemtion option does not make the platform code > ready. > Let's understand your crash case. Let's see where it is going wrong. I am suspecting it is some wrong usage of preemption api rather than arch can't support preemption. > Thanks > > Michal > >> >> Christophe >> >>>> >>>> >>>>> >>>>> Currently we use the following revert patch for our non-preempt kernels. >>>>> >>>>> --- a/kernel/Kconfig.preempt    2026-02-14 10:06:32.000000000 +0000 >>>>> +++ b/kernel/Kconfig.preempt    2026-02-08 21:03:27.000000000 +0000 >>>>> @@ -16,13 +16,11 @@ config ARCH_HAS_PREEMPT_LAZY >>>>> >>>>>   choice >>>>>       prompt "Preemption Model" >>>>> -    default PREEMPT_LAZY if ARCH_HAS_PREEMPT_LAZY >>>>>       default PREEMPT_NONE >>>>> >>>>>   config PREEMPT_NONE >>>>>       bool "No Forced Preemption (Server)" >>>>>       depends on !PREEMPT_RT >>>>> -    depends on ARCH_NO_PREEMPT >>>>>       select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC >>>>>       help >>>>>         This is the traditional Linux preemption model, geared towards >>>>> @@ -37,7 +35,6 @@ config PREEMPT_NONE >>>>> >>>>>   config PREEMPT_VOLUNTARY >>>>>       bool "Voluntary Kernel Preemption (Desktop)" >>>>> -    depends on !ARCH_HAS_PREEMPT_LAZY >>>>>       depends on !ARCH_NO_PREEMPT >>>>>       depends on !PREEMPT_RT >>>>>       select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC >>>>> >>>>> --- >>>>> >>>>> Thanks in advance, >>>>> >>>>> Christian >>>>> >>>> >>> >>> >>