linux-kernel - Re: [PATCH v4 4/5] cpumask: Add UP optimised for_each_*

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Date:   Sat, 2 Jul 2022 14:47:54 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Sander Vanheule <sander@...nheule.net>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        elver@...gle.com, gregkh@...uxfoundation.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, vschneid@...hat.com,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v4 4/5] cpumask: Add UP optimised for_each_*_cpu versions

On Sat, Jul 02, 2022 at 06:08:27PM +0200, Sander Vanheule wrote:
> On uniprocessor builds, the following loops will always run over a mask
> that contains one enabled CPU (cpu0):
>     - for_each_possible_cpu
>     - for_each_online_cpu
>     - for_each_present_cpu
> 
> Provide uniprocessor-specific macros for these loops, that always run
> exactly once.
> 
> Signed-off-by: Sander Vanheule <sander@...nheule.net>

Acked-by: Yury Norov <yury.norov@...il.com>

> ---
>  include/linux/cpumask.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index 7fbef41b3093..6c5b4ee000f2 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -750,9 +750,16 @@ extern const DECLARE_BITMAP(cpu_all_bits, NR_CPUS);
>  /* First bits of cpu_bit_bitmap are in fact unset. */
>  #define cpu_none_mask to_cpumask(cpu_bit_bitmap[0])
>  
> +#if NR_CPUS == 1
> +/* Uniprocessor: the possible/online/present masks are always "1" */
> +#define for_each_possible_cpu(cpu)	for ((cpu) = 0; (cpu) < 1; (cpu)++)
> +#define for_each_online_cpu(cpu)	for ((cpu) = 0; (cpu) < 1; (cpu)++)
> +#define for_each_present_cpu(cpu)	for ((cpu) = 0; (cpu) < 1; (cpu)++)
> +#else
>  #define for_each_possible_cpu(cpu) for_each_cpu((cpu), cpu_possible_mask)
>  #define for_each_online_cpu(cpu)   for_each_cpu((cpu), cpu_online_mask)
>  #define for_each_present_cpu(cpu)  for_each_cpu((cpu), cpu_present_mask)
> +#endif
>  
>  /* Wrappers for arch boot code to manipulate normally-constant masks */
>  void init_cpu_present(const struct cpumask *src);
> -- 
> 2.36.1