Re: [patch V5 09/20] cpumask: Cache num_possible_cpus()

[Marek Szyprowski]

On 22.11.2025 16:36, Thomas Gleixner wrote:
> On Fri, Nov 21 2025 at 23:56, Marek Szyprowski wrote:
>> Reverting it on top of linux-next fixes the issue. Let me know how can I
>> help debugging it.
> Can you test the fix below please?

This fixes the observed issue. Thanks!

Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

> Thanks,
>
>          tglx
> ---
> Subject: cpu: Initialize __num_possible_cpus correctly
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Date: Sat, 22 Nov 2025 16:19:18 +0100
>
> The variable to cache the number of possible CPUs is initialized to NR_CPUS
> at build time, but that's only correct when cpu_possible_mask is
> initialized with CPU_BITS_ALL. That's only the case on PARISC.
>
> On x86 and some other architectures this does not matter because they
> initialize cpu_possible_mask via init_cpu_possible() which does a proper
> weight calculation. Though on architectures which do not, this results
> in a completely wrong cached value 'NR_CPUS + actual possible CPUs'.
>
> Initialize it correctly to 0 when CONFIG_INIT_ALL_POSSIBLE=n and move the
> NR_CPUS initialization into the PARISC specific section.
>
> Fixes: d0f23ccf6ba9 ("cpumask: Cache num_possible_cpus()")
> Reported-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/89c7106e-a431-443a-9527-3d5fbce77fe1@xxxxxxxxxxx
> Closes: https://lore.kernel.org/all/20251122002755.GA2682494@ax162
> ---
>   kernel/cpu.c |    6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -3085,10 +3085,13 @@ EXPORT_SYMBOL(cpu_all_bits);
>   #ifdef CONFIG_INIT_ALL_POSSIBLE
>   struct cpumask __cpu_possible_mask __ro_after_init
>   	= {CPU_BITS_ALL};
> +unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
>   #else
>   struct cpumask __cpu_possible_mask __ro_after_init;
> +unsigned int __num_possible_cpus __ro_after_init;
>   #endif
>   EXPORT_SYMBOL(__cpu_possible_mask);
> +EXPORT_SYMBOL(__num_possible_cpus);
>   
>   struct cpumask __cpu_online_mask __read_mostly;
>   EXPORT_SYMBOL(__cpu_online_mask);
> @@ -3108,9 +3111,6 @@ EXPORT_SYMBOL(__cpu_dying_mask);
>   atomic_t __num_online_cpus __read_mostly;
>   EXPORT_SYMBOL(__num_online_cpus);
>   
> -unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
> -EXPORT_SYMBOL(__num_possible_cpus);
> -
>   void init_cpu_present(const struct cpumask *src)
>   {
>   	cpumask_copy(&__cpu_present_mask, src);
>
Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland