linux-kernel - Re: [PATCH 4/4] x86/resctrl: optimize cpumask_any

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Message-ID: <de0758c8-76d5-4ead-84f2-40db45ff4ed8@intel.com>
Date: Wed, 23 Apr 2025 14:29:48 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Yury Norov <yury.norov@...il.com>, Tony Luck <tony.luck@...el.com>,
	"Thomas Gleixner" <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	"Borislav Petkov" <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Rasmus Villemoes
	<linux@...musvillemoes.dk>, <x86@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] x86/resctrl: optimize cpumask_any_housekeeping()

Hi Yury,

On 4/7/25 8:38 AM, Yury Norov wrote:
> From: Yury Norov [NVIDIA] <yury.norov@...il.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@...il.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