Re: [RFC PATCH v4 5/8] x86/smp native_play_dead: Prefer cpuidle_play_dead() over mwait_play_dead()

From: Peter Zijlstra
Date: Mon Nov 25 2024 - 08:45:19 EST


On Mon, Nov 25, 2024 at 02:20:25PM +0100, Patryk Wlazlyn wrote:
> The algorithm based on cpuid leaf 0x5 in the mwait_play_dead(),
> for looking up the mwait hint for the deepest cstate may calculate the
> wrong hint on recent Intel x86 platforms.
>
> Prefer cpuidle_play_dead() over the generic mwait_play_dead() loop and
> only fallback to the later in case of missing appropriate enter_dead()
> handler.

So 7/8 should go first, such that picking cpuidle doesn't end up in some
weird state.

>
> Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@xxxxxxxxxxxxxxx>
> ---
> arch/x86/kernel/smpboot.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index d0464c7a0af5..2627b56fb9bc 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1413,9 +1413,10 @@ void native_play_dead(void)
> play_dead_common();
> tboot_shutdown(TB_SHUTDOWN_WFS);
>
> + /* The first successful play_dead() will not return */
> + cpuidle_play_dead();
> mwait_play_dead();
> - if (cpuidle_play_dead())
> - hlt_play_dead();
> + hlt_play_dead();
> }
>
> #else /* ... !CONFIG_HOTPLUG_CPU */
> --
> 2.47.0
>