linux-kernel - Re: [patch 07/19] cpumask: Introduce cpumask_or

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Message-ID: <aO_iiKKVyKSlXeF2@yury>
Date: Wed, 15 Oct 2025 14:06:00 -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>,
	TCMalloc Team <tcmalloc-eng@...gle.com>
Subject: Re: [patch 07/19] cpumask: Introduce cpumask_or_weight()

On Wed, Oct 15, 2025 at 01:41:50PM -0400, Yury Norov wrote:
> Hi Tomas,
> 
> On Wed, Oct 15, 2025 at 07:29:36PM +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.
> > 
> > Provide cpumask_or_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>
> > ---
> >  include/linux/bitmap.h  |   15 +++++++++++++++
> >  include/linux/cpumask.h |   16 ++++++++++++++++
> >  lib/bitmap.c            |   17 +++++++++++++++++
> >  3 files changed, 48 insertions(+)
> > 
> > --- a/include/linux/bitmap.h
> > +++ b/include/linux/bitmap.h
> > @@ -45,6 +45,7 @@ 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_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 +166,8 @@ 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_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 +341,18 @@ void bitmap_or(unsigned long *dst, const
> >  }
> >  
> >  static __always_inline
> > +unsigned int bitmap_or_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_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_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_weight(struct cpumask *dstp, const struct cpumask *src1p,
> > +			       const struct cpumask *src2p)
> > +{
> > +	return bitmap_or_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
> > @@ -253,6 +253,23 @@ void __bitmap_or(unsigned long *dst, con
> >  }
> >  EXPORT_SYMBOL(__bitmap_or);
> >  
> > +unsigned int __bitmap_or_weight(unsigned long *dst, const unsigned long *bitmap1,
> > +				const unsigned long *bitmap2, unsigned int bits)
> > +{
> > +	unsigned int k, w = 0;
> > +
> > +	for (k = 0; k < bits / BITS_PER_LONG; k++) {
> > +		dst[k] = bitmap1[k] | bitmap2[k];
> > +		w += hweight_long(dst[k]);
> > +	}
> > +
> > +	if (bits % BITS_PER_LONG) {
> > +		dst[k] = bitmap1[k] | bitmap2[k];
> > +		w += hweight_long(dst[k] & BITMAP_LAST_WORD_MASK(bits));
> > +	}
> > +	return w;
> > +}
> 
> We've got bitmap_weight_and() and bitmap_weight_andnot() already. Can
> you align naming with the existing scheme: bitmap_weight_or().
> 
> Also, for outline implementation, can you employ the BITMAP_WEIGHT()
> macro?

Ok, I see now. You want to do a regular cpumask_or(), but return the
hweight() of the result, instead of a boolean.

The cpumask_or_weight() may be really confused with cpumask_weight_or().
Can you try considering a different naming? (I am seemingly can't.)

Can you describe the performance impact you've mentioned in the commit
message in more details?

Anyways, for the approach:

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