Re: [PATCH v2 0/6] sched: dynamic: Simplify PREEMPT_DYNAMIC

[Mark Rutland]

On Fri, Aug 07, 2026 at 11:33:05AM +0800, Jinjie Ruan wrote:
> 
> 
> 在 2026/8/4 3:17, Mark Rutland 写道:
> > All architectures which currently suppoort PREEMPT_DYNAMIC select
> > ARCH_HAS_PREEMPT_LAZY.  On architectures which select
> > ARCH_HAS_PREEMPT_LAZY, it has not been possible to select the NONE and
> > VOLUNTARY preemption models since v7.0 due to commit:
> > 
> >   7dadeaa6e851 ("sched: Further restrict the preemption modes")
> > 
> > Hence in practice PREEMPT_DYNAMIC no longer supports the NONE or
> > VOLUNTARY preemption models.
> > 
> > This series makes the de-facto situation official by making
> > PREEMPT_DYNAMIC depend on ARCH_HAS_PREEMPT_LAZY, and removing redundant
> > code.
> > 
> > Since v1, I've had to rebase atop v7.2-rc4 to avoid a conflict with
> > commit:
> > 
> >   a2c02aa0c6ca ("powerpc: Remove dead non-preemption code")
> > 
> > Given this is now late in the cycle, and given the tip:sched/core branch
> > is based on v7.2-rc1, I suspect tip folk might not want to queue this
> > now. Please let me know what you'd prefer; I'm happy to rebase and
> > resend after v7.3-rc1.
> > 
> > Since v1:
> > * Rebase to v7.2-rc4
> > * Apply Revied-by tags.
> > * Fix typos in commit messages.
> > * Add patch to cleanup accessors (per Shrikanth's comment)
> 
> Hi Mark,
> 
> I did a test on v7.2-rc6 for arm64.
> 
> # ls -la Image_base  Image_patch
> -rw-r--r-- 1 root root 63158784 Aug  7 10:37 Image_base
> -rw-r--r-- 1 root root 63093248 Aug  7 10:31 Image_patch
> # ls -la vmlinux_base vmlinux_patch
> -rwxr-xr-x 1 root root 206396648 Aug  7 10:37 vmlinux_base
> -rwxr-xr-x 1 root root 206266232 Aug  7 10:31 vmlinux_patch
> 
> So the overall vmlinux file size reduced by ~127 KB and the Image
> size reduced by ~64 KB.
> 
> Boot tested successfully with preempt=lazy and preempt=full on Qemu and
> Kunpeng HIP09 server.
> 
> Boot with preempt=none and preempt=voluntary on Qemu.
> 
>     Dynamic Preempt: unsupported mode: none
>     Dynamic Preempt: unsupported mode: voluntary
> 
> But runtime switching via /sys/kernel/debug/sched/preempt works not
> right as below.
> 
> # echo full > preempt
> # cat preempt
> full lazy
> # echo lazy > preempt
> # cat preempt
> full lazy
> 
> We should also update the "preempt_modes" array, after the below fix, it
> shows ok;
> 
> # echo lazy > preempt
> [   37.407616] Dynamic Preempt: lazy
> # cat preempt
> full (lazy)
> # echo full > preempt
> [   52.067001] Dynamic Preempt: full
> # cat preempt
> (full) lazy
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7899,7 +7899,7 @@ static inline void preempt_dynamic_init(void) { }
>  #endif /* CONFIG_PREEMPT_DYNAMIC */
> 
>  const char *preempt_modes[] = {
> -       "none", "voluntary", "full", "lazy", NULL,
> +       "full", "lazy", NULL,
>  };

Ugh, yes.

We'll also need to fix up sched_dynamic_show(), since it has logic to
conditionally skip "none" and "voluntary".

This is all a bit messy, given the enum value isn't used to define the
string array. I'll see if I can figure a way to make that consistent by
construction, but that probably needs a bit more rework...

> Otherwise
> Tested-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>

Thanks!

Mark.