Re: [PATCH] Revert "intel_idle: Rescan "dead" SMT siblings during, initialization"

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Aug 28, 2025 at 4:26 PM Christian Loehle
<christian.loehle@arm.com> wrote:
>
> This reverts commit a430c11f401589a0f4f57fd398271a5d85142c7a.
>
> Calling arch_cpu_rescan_dead_smt_siblings() in intel_idle_init with
> boot parameter nosmt and maxcpus active hotplugged boot-offline CPUs
> in (and leave them online) which weren't supposed to be online.
>
> With the revert and nosmt and maxcpus=12 on a raptor lake:
> cpu     online  capacity
> cpu0    1       1009
> cpu1    0       -
> cpu2    1       1009
> cpu3    0       -
> cpu4    1       1009
> cpu5    0       -
> cpu6    1       1009
> cpu7    0       -
> cpu8    1       1024
> cpu9    0       -
> cpu10   1       1024
> cpu11   0       -
> cpu12   1       1009
> cpu13   0       -
> cpu14   1       1009
> cpu15   0       -
> cpu16   1       623
> cpu17   1       623
> cpu18   1       623
> cpu19   1       623
> cpu20   0       -
> cpu21   0       -
> cpu22   0       -
> cpu23   0       -
>
> Previously:
> cpu     online  capacity
> cpu0    1       1009
> cpu1    0       -
> cpu2    1       1009
> cpu3    0       -
> cpu4    1       1009
> cpu5    0       -
> cpu6    1       1009
> cpu7    0       -
> cpu8    1       1024
> cpu9    0       -
> cpu10   1       1024
> cpu11   0       -
> cpu12   1       1009
> cpu13   0       -
> cpu14   1       1009
> cpu15   0       -
> cpu16   1       623
> cpu17   1       623
> cpu18   1       623
> cpu19   1       623
> cpu20   1       623
> cpu21   1       623
> cpu22   1       623
> cpu23   1       623
>
> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
> ---
> Rafael, I don't immediately see how to fix this properly so I won't
> try to, feel free to treat this as a bug report.

Sure, thanks for reporting this!

Well, I think that cpuhp_smt_enable() is missing a check.  It looks to
me like it should do the topology_is_primary_thread(cpu) check like
cpuhp_smt_disable().

I'll cut a test patch for this later.

>  drivers/idle/intel_idle.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index 91a7b7e7c0c8..a9c58395a425 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -2507,8 +2507,6 @@ static int __init intel_idle_init(void)
>         pr_debug("Local APIC timer is reliable in %s\n",
>                  boot_cpu_has(X86_FEATURE_ARAT) ? "all C-states" : "C1");
>
> -       arch_cpu_rescan_dead_smt_siblings();
> -
>         return 0;
>
>  hp_setup_fail:
> --