linux-kernel - Re: [patch V2 07/20] cpumask: Introduce cpumask_or_and_calc

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Message-ID: <aPpZ1HHXh_RMGIjR@yury>
Date: Thu, 23 Oct 2025 12:37:40 -0400
From: Yury Norov <yury.norov@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Gabriele Monaco <gmonaco@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Michael Jeanson <mjeanson@...icios.com>,
	Jens Axboe <axboe@...nel.dk>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>,
	Florian Weimer <fweimer@...hat.com>,
	Tim Chen <tim.c.chen@...el.com>
Subject: Re: [patch V2 07/20] cpumask: Introduce cpumask_or_and_calc_weight()

On Wed, Oct 22, 2025 at 02:55:28PM +0200, Thomas Gleixner wrote:
> CID management OR's two cpumasks and then calculates the weight on the
> result. That's inefficient as that has to walk the same stuff twice. As
> this is done with runqueue lock held, there is a real benefit of speeding
> this up. Depending on the system this results in 10-20% less cycles spent
> with runqueue lock held for a 4K cpumask.
> 
> Provide cpumask_or_and_calc_weight() and the corresponding bitmap functions
> which return the weight of the OR result right away.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Yury Norov <yury.norov@...il.com>
> ---
> V2: Rename and use the BITMAP_WEIGHT() macro - Yury
> ---
>  include/linux/bitmap.h  |   16 ++++++++++++++++
>  include/linux/cpumask.h |   16 ++++++++++++++++
>  lib/bitmap.c            |    6 ++++++
>  3 files changed, 38 insertions(+)
> 
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -45,6 +45,8 @@ struct device;
>   *  bitmap_copy(dst, src, nbits)                *dst = *src
>   *  bitmap_and(dst, src1, src2, nbits)          *dst = *src1 & *src2
>   *  bitmap_or(dst, src1, src2, nbits)           *dst = *src1 | *src2
> + *  bitmap_or_and_calc_weight(dst, src1, src2, nbits)
> + *						*dst = *src1 | *src2. Returns Hamming Weight of dst
>   *  bitmap_xor(dst, src1, src2, nbits)          *dst = *src1 ^ *src2
>   *  bitmap_andnot(dst, src1, src2, nbits)       *dst = *src1 & ~(*src2)
>   *  bitmap_complement(dst, src, nbits)          *dst = ~(*src)
> @@ -165,6 +167,8 @@ bool __bitmap_and(unsigned long *dst, co
>  		 const unsigned long *bitmap2, unsigned int nbits);
>  void __bitmap_or(unsigned long *dst, const unsigned long *bitmap1,
>  		 const unsigned long *bitmap2, unsigned int nbits);
> +unsigned int __bitmap_or_and_calc_weight(unsigned long *dst, const unsigned long *bitmap1,
> +					 const unsigned long *bitmap2, unsigned int nbits);
>  void __bitmap_xor(unsigned long *dst, const unsigned long *bitmap1,
>  		  const unsigned long *bitmap2, unsigned int nbits);
>  bool __bitmap_andnot(unsigned long *dst, const unsigned long *bitmap1,
> @@ -338,6 +342,18 @@ void bitmap_or(unsigned long *dst, const
>  }
>  
>  static __always_inline
> +unsigned int bitmap_or_and_calc_weight(unsigned long *dst, const unsigned long *src1,
> +				       const unsigned long *src2, unsigned int nbits)
> +{
> +	if (small_const_nbits(nbits)) {
> +		*dst = *src1 | *src2;
> +		return hweight_long(*dst & BITMAP_LAST_WORD_MASK(nbits));
> +	} else {
> +		return __bitmap_or_and_calc_weight(dst, src1, src2, nbits);
> +	}
> +}
> +
> +static __always_inline
>  void bitmap_xor(unsigned long *dst, const unsigned long *src1,
>  		const unsigned long *src2, unsigned int nbits)
>  {
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -729,6 +729,22 @@ void cpumask_or(struct cpumask *dstp, co
>  }
>  
>  /**
> + * cpumask_or_and_calc_weight - *dstp = *src1p | *src2p and return the weight of the result
> + * @dstp: the cpumask result
> + * @src1p: the first input
> + * @src2p: the second input
> + *
> + * Return: The number of bits set in the resulting cpumask @dstp
> + */
> +static __always_inline
> +unsigned int cpumask_or_and_calc_weight(struct cpumask *dstp, const struct cpumask *src1p,
> +					const struct cpumask *src2p)
> +{
> +	return bitmap_or_and_calc_weight(cpumask_bits(dstp), cpumask_bits(src1p),
> +					 cpumask_bits(src2p), small_cpumask_bits);
> +}
> +
> +/**
>   * cpumask_xor - *dstp = *src1p ^ *src2p
>   * @dstp: the cpumask result
>   * @src1p: the first input
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -355,6 +355,12 @@ unsigned int __bitmap_weight_andnot(cons
>  }
>  EXPORT_SYMBOL(__bitmap_weight_andnot);
>  
> +unsigned int __bitmap_or_and_calc_weight(unsigned long *dst, const unsigned long *bitmap1,
> +					 const unsigned long *bitmap2, unsigned int bits)
> +{
> +	return BITMAP_WEIGHT(({dst[idx] = bitmap1[idx] | bitmap2[idx]; dst[idx]; }), bits);

Nice!

> +}
> +

Reviewed-by: Yury Norov (NVIDIA) <yury.norov@...il.com>

Maybe bitmap_weighted_or()?