linux-kernel - Re: [PATCH v1 2/2] bitmap: Optimize memset() calls

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Message-ID: <e66919a4-afdc-efdb-f6ae-07cfc5bde105@rasmusvillemoes.dk>
Date:   Fri, 18 Aug 2023 08:53:38 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] bitmap: Optimize memset() calls

On 17/08/2023 18.54, Andy Shevchenko wrote:
> Intead of byte memset() calls use 32- or 64-bit version depending
> on the compile-time BITS_PER_LONG value.
> 
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>  include/linux/bitmap.h | 16 ++++++++++++----
>  lib/bitmap.c           |  4 ++--
>  2 files changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 2d5042d1b501..6eec4d4fd623 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -234,22 +234,30 @@ extern int bitmap_print_list_to_buf(char *buf, const unsigned long *maskp,
>  
>  static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
>  {
> -	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> +	unsigned int len = BITS_TO_LONGS(nbits);
>  
>  	if (small_const_nbits(nbits))
>  		*dst = 0;
>  	else
> -		memset(dst, 0, len);
> +#if BITS_PER_LONG == 64
> +		memset64((uint64_t *)dst, 0, len);
> +#else
> +		memset32((uint32_t *)dst, 0, len);
> +#endif
>  }
>  

So _if_ this is worth it at all, all those new '#if BITS_PER_LONG == 64'
suggests that we should instead have a new helper memset_long(), no?

In fact, string.h already has that:

static inline void *memset_l(unsigned long *p, unsigned long v,
                __kernel_size_t n)


>  static inline void bitmap_fill(unsigned long *dst, unsigned int nbits)
>  {
> -	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> +	unsigned int len = BITS_TO_LONGS(nbits);
>  
>  	if (small_const_nbits(nbits))
>  		*dst = ~0UL;
>  	else
> -		memset(dst, 0xff, len);
> +#if BITS_PER_LONG == 64
> +		memset64((uint64_t *)dst, GENMASK(63, 0), len);
> +#else
> +		memset32((uint32_t *)dst, GENMASK(31, 0), len);
> +#endif>  }
>

Please just spell an all-ones long "~0UL", that also matches the
small_const case.


>  static inline void bitmap_copy(unsigned long *dst, const unsigned long *src,
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index 935e0f96e785..df0fb37a5732 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -128,7 +128,7 @@ void __bitmap_shift_right(unsigned long *dst, const unsigned long *src,
>  		dst[k] = lower | upper;
>  	}
>  	if (off)
> -		memset(&dst[lim - off], 0, off*sizeof(unsigned long));
> +		bitmap_zero(&dst[lim - off], off);

This... can't be right. bitmap_zero() still takes an argument which is
the size in bits, while off is the whole number of words to shift. So if
called with say shift=128, we'd have off==2, and that bitmap_zero()
would, because bitmap_zero() rounds up to a whole number of words, end
up clearing just one word.

Perhaps a chance to add some more test cases? Maybe we're not exercising
any of the "shift more than BITS_PER_LONG" logic.

But rather than using bitmap_zero() here, forcing you to convert off to
a number of bits and then bitmap_zero to divide again, if you do this at
all, just change that memset() to memset_l().

>  }
>  EXPORT_SYMBOL(__bitmap_shift_right);
>  
> @@ -166,7 +166,7 @@ void __bitmap_shift_left(unsigned long *dst, const unsigned long *src,
>  		dst[k + off] = lower | upper;
>  	}
>  	if (off)
> -		memset(dst, 0, off*sizeof(unsigned long));
> +		bitmap_zero(dst, off);
>  }

Same here. Cannot possibly be correct, but will work by chance for off
<= 1, i.e. shift <= BITS_PER_LONG.

Rasmus