Re: [PATCH v6 1/5] lib/bitmap: add bitmap_{read,write}()

[Yury Norov <yury.norov@gmail.com>]

On Fri, Oct 06, 2023 at 03:45:25PM +0200, Alexander Potapenko wrote:
> From: Syed Nayyar Waris <syednwaris@gmail.com>
> 
> The two new functions allow reading/writing values of length up to
> BITS_PER_LONG bits at arbitrary position in the bitmap.
> 
> The code was taken from "bitops: Introduce the for_each_set_clump macro"
> by Syed Nayyar Waris with a number of changes and simplifications:
>  - instead of using roundup(), which adds an unnecessary dependency
>    on <linux/math.h>, we calculate space as BITS_PER_LONG-offset;
>  - indentation is reduced by not using else-clauses (suggested by
>    checkpatch for bitmap_get_value());
>  - bitmap_get_value()/bitmap_set_value() are renamed to bitmap_read()
>    and bitmap_write();
>  - some redundant computations are omitted.
> 
> Cc: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Syed Nayyar Waris <syednwaris@gmail.com>
> Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
> Link: https://lore.kernel.org/lkml/fe12eedf3666f4af5138de0e70b67a07c7f40338.1592224129.git.syednwaris@gmail.com/
> Suggested-by: Yury Norov <yury.norov@gmail.com>
> Co-developed-by: Alexander Potapenko <glider@google.com>
> Signed-off-by: Alexander Potapenko <glider@google.com>
> 
> ---
> This patch was previously called "lib/bitmap: add
> bitmap_{set,get}_value()"
> (https://lore.kernel.org/lkml/20230720173956.3674987-2-glider@google.com/)
> 
> v6:
>  - As suggested by Yury Norov, do not require bitmap_read(..., 0) to
>    return 0.
> 
> v5:
>  - Address comments by Yury Norov:
>    - updated code comments and patch title/description
>    - replace GENMASK(nbits - 1, 0) with BITMAP_LAST_WORD_MASK(nbits)
>    - more compact bitmap_write() implementation
> 
> v4:
>  - Address comments by Andy Shevchenko and Yury Norov:
>    - prevent passing values >= 64 to GENMASK()
>    - fix commit authorship
>    - change comments
>    - check for unlikely(nbits==0)
>    - drop unnecessary const declarations
>    - fix kernel-doc comments
>    - rename bitmap_{get,set}_value() to bitmap_{read,write}()
> ---
>  include/linux/bitmap.h | 68 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 68 insertions(+)
> 
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index 03644237e1efb..e72c054d21d48 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -76,7 +76,11 @@ struct device;
>   *  bitmap_to_arr32(buf, src, nbits)            Copy nbits from buf to u32[] dst
>   *  bitmap_to_arr64(buf, src, nbits)            Copy nbits from buf to u64[] dst
>   *  bitmap_get_value8(map, start)               Get 8bit value from map at start
> + *  bitmap_read(map, start, nbits)              Read an nbits-sized value from
> + *                                              map at start
>   *  bitmap_set_value8(map, value, start)        Set 8bit value to map at start
> + *  bitmap_write(map, value, start, nbits)      Write an nbits-sized value to
> + *                                              map at start
>   *
>   * Note, bitmap_zero() and bitmap_fill() operate over the region of
>   * unsigned longs, that is, bits behind bitmap till the unsigned long
> @@ -583,6 +587,33 @@ static inline unsigned long bitmap_get_value8(const unsigned long *map,
>  	return (map[index] >> offset) & 0xFF;
>  }
>  
> +/**
> + * bitmap_read - read a value of n-bits from the memory region
> + * @map: address to the bitmap memory region
> + * @start: bit offset of the n-bit value
> + * @nbits: size of value in bits, nonzero, up to BITS_PER_LONG
> + *
> + * Returns: value of nbits located at the @start bit offset within the @map
> + * memory region.
> + */
> +static inline unsigned long bitmap_read(const unsigned long *map,
> +					unsigned long start,
> +					unsigned long nbits)
> +{
> +	size_t index = BIT_WORD(start);
> +	unsigned long offset = start % BITS_PER_LONG;
> +	unsigned long space = BITS_PER_LONG - offset;
> +	unsigned long value_low, value_high;
> +
> +	if (unlikely(!nbits))
> +		return 0;
> +	if (space >= nbits)
> +		return (map[index] >> offset) & GENMASK(nbits - 1, 0);
> +	value_low = map[index] & BITMAP_FIRST_WORD_MASK(start);
> +	value_high = map[index + 1] & BITMAP_LAST_WORD_MASK(start + nbits);
> +	return (value_low >> offset) | (value_high << space);
> +}
> +
>  /**
>   * bitmap_set_value8 - set an 8-bit value within a memory region
>   * @map: address to the bitmap memory region
> @@ -599,6 +630,43 @@ static inline void bitmap_set_value8(unsigned long *map, unsigned long value,
>  	map[index] |= value << offset;
>  }
>  
> +/**
> + * bitmap_write - write n-bit value within a memory region
> + * @map: address to the bitmap memory region
> + * @value: value to write, clamped to nbits
> + * @start: bit offset of the n-bit value
> + * @nbits: size of value in bits, nonzero, up to BITS_PER_LONG.
> + *
> + * bitmap_write() behaves similarly to @nbits calls of assign_bit(), i.e. bits
> + * beyond @nbits are ignored:
> + *
> + *   for (bit = 0; bit < nbits; bit++)
> + *           assign_bit(start + bit, bitmap, val & BIT(bit));

__assign_bit()

> + */

'behaves similarly' sounds like an understatement. I think, it behaves
much faster because it can assign up to 64 bits at once, not mentioning
the pressure on cache lines traffic.

How faster - that's a good question. I'd be really pleased if you add
a performance test for bitmap_write/read. Or I can do it myself later.
You can find examples in the same lib/test_bitmap.c.

> +static inline void bitmap_write(unsigned long *map,
> +				unsigned long value,
> +				unsigned long start, unsigned long nbits)
> +{
> +	size_t index = BIT_WORD(start);
> +	unsigned long offset = start % BITS_PER_LONG;
> +	unsigned long space = BITS_PER_LONG - offset;
> +	unsigned long mask;
> +
> +	if (unlikely(!nbits))
> +		return;

can you please add more empty lines to separate blocks visually?

> +	mask = BITMAP_LAST_WORD_MASK(nbits);
> +	value &= mask;
> +	if (space >= nbits) {
> +		map[index] &= ~(mask << offset);
> +		map[index] |= value << offset;
> +		return;
> +	}
> +	map[index] &= ~BITMAP_FIRST_WORD_MASK(start);
> +	map[index] |= value << offset;
> +	map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> +	map[index + 1] |= (value >> space);
> +}

I compiled the below fix on spark64 BE machine:

--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -608,7 +608,7 @@ static inline unsigned long bitmap_read(const unsigned long *map,
        if (unlikely(!nbits))
                return 0;
        if (space >= nbits)
-               return (map[index] >> offset) & GENMASK(nbits - 1, 0);
+               return (map[index] >> offset) & BITMAP_LAST_WORD_MASK(nbits);
        value_low = map[index] & BITMAP_FIRST_WORD_MASK(start);
        value_high = map[index + 1] & BITMAP_LAST_WORD_MASK(start + nbits);
        return (value_low >> offset) | (value_high << space);
@@ -661,9 +661,9 @@ static inline void bitmap_write(unsigned long *map,
                map[index] |= value << offset;
                return;
        }
-       map[index] &= ~BITMAP_FIRST_WORD_MASK(start);
+       map[index] &= BITMAP_LAST_WORD_MASK(start);
        map[index] |= value << offset;
-       map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
+       map[index + 1] &= BITMAP_FIRST_WORD_MASK(start + nbits);
        map[index + 1] |= (value >> space);
 }

All the tests are passed just as before, and there's no any difference
reported by bloat-o-meter. Can you please use non-negation versions as
they are more straightforward?

> +
>  #endif /* __ASSEMBLY__ */
>  
>  #endif /* __LINUX_BITMAP_H */
> -- 
> 2.42.0.609.gbb76f46606-goog