Re: [PATCH 4/4] x86/resctrl: optimize cpumask_any_housekeeping()

Reinette Chatre
2025-04-23T21:29:56Z

Hi Yury,

On 4/7/25 8:38 AM, Yury Norov wrote:
> From: Yury Norov [NVIDIA] <yury.norov@gmail.com>
>
> With the lack of cpumask_andnot_any_but(), users have to abuse
> cpumask_nth() functions which are O(N*log(N)), comparing to O(N)
> for cpumask_any().
>
> This series adds missing cpumask_andnot_any_but() and makes
> cpumask_any_but() understanding the RESCTRL_PICK_ANY_CPU hint.
> This simplifies cpumask_any_housekeeping() significantly.

The "This series ..." language is more appropriate for the cover
letter.

This changelog could be something like:

With the lack of cpumask_andnot_any_but(), cpumask_any_housekeeping()
abused cpumask_nth() functions which are O(N*log(N)), compared to O(N)
for cpumask_any().

Update cpumask_any_housekeeping() to use the new cpumask_any_but() and
cpumask_andnot_any_but(). These two functions understand RESCTRL_PICK_ANY_CPU
and simplifies cpumask_any_housekeeping() significantly.

Also, could you please have the subject of this patch start with an
upper case: "x86/resctrl: Optimize cpumask_any_housekeeping()"?

>
> Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> ---
> arch/x86/kernel/cpu/resctrl/internal.h | 28 +++++++-------------------
> 1 file changed, 7 insertions(+), 21 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
> index 20c898f09b7e..1db02bab9743 100644
> --- a/arch/x86/kernel/cpu/resctrl/internal.h
> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
> @@ -71,30 +71,16 @@
> static inline unsigned int
> cpumask_any_housekeeping(const struct cpumask *mask, int exclude_cpu)
> {
> - unsigned int cpu, hk_cpu;
> -
> - if (exclude_cpu == RESCTRL_PICK_ANY_CPU)
> - cpu = cpumask_any(mask);
> - else
> - cpu = cpumask_any_but(mask, exclude_cpu);
> -
> - /* Only continue if tick_nohz_full_mask has been initialized. */
> - if (!tick_nohz_full_enabled())
> - return cpu;
> -
> - /* If the CPU picked isn't marked nohz_full nothing more needs doing. */
> - if (cpu < nr_cpu_ids && !tick_nohz_full_cpu(cpu))
> - return cpu;
> + unsigned int cpu;
>
> /* Try to find a CPU that isn't nohz_full to use in preference */
> - hk_cpu = cpumask_nth_andnot(0, mask, tick_nohz_full_mask);
> - if (hk_cpu == exclude_cpu)
> - hk_cpu = cpumask_nth_andnot(1, mask, tick_nohz_full_mask);
> -
> - if (hk_cpu < nr_cpu_ids)
> - cpu = hk_cpu;
> + if (tick_nohz_full_enabled()) {
> + cpu = cpumask_andnot_any_but(mask, tick_nohz_full_mask, exclude_cpu);
> + if (cpu < nr_cpu_ids)
> + return cpu;
> + }
>
> - return cpu;
> + return cpumask_any_but(mask, exclude_cpu);
> }
>
> struct rdt_fs_context {

This looks good to me. Thank you very much for doing this.

Reinette