linux-kernel - Re: [PATCH v4 3/4] lib/find_bit: optimize find_next

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Date:   Thu, 15 Sep 2022 17:25:24 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>,
        Alexey Klimov <klimov.linux@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andy Whitcroft <apw@...onical.com>,
        Catalin Marinas <catalin.marinas@....com>,
        David Laight <David.Laight@...LAB.COM>,
        Dennis Zhou <dennis@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Kees Cook <keescook@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH v4 3/4] lib/find_bit: optimize find_next_bit() functions

On 14/09/22 19:07, Yury Norov wrote:
> Over the past couple years, the function _find_next_bit() was extended
> with parameters that modify its behavior to implement and- zero- and le-
> flavors. The parameters are passed at compile time, but current design
> prevents a compiler from optimizing out the conditionals.
>
> As find_next_bit() API grows, I expect that more parameters will be added.
> Current design would require more conditional code in _find_next_bit(),
> which would bloat the helper even more and make it barely readable.
>
> This patch replaces _find_next_bit() with a macro FIND_NEXT_BIT, and adds
> a set of wrappers, so that the compile-time optimizations become possible.
>
> The common logic is moved to the new macro, and all flavors may be
> generated by providing a FETCH macro parameter, like in this example:
>
>   #define FIND_NEXT_BIT(FETCH, MUNGE, size, start) ...
>
>   find_next_xornot_and_bit(addr1, addr2, addr3, size, start)
>   {
>       return FIND_NEXT_BIT(addr1[idx] ^ ~addr2[idx] & addr3[idx],
>                               /* nop */, size, start);
>   }
>
> The FETCH may be of any complexity, as soon as it only refers the bitmap(s)
> and an iterator idx.
>
> MUNGE is here to support _le code generation for BE builds. May be
> empty.
>
> I ran find_bit_benchmark 16 times on top of 6.0-rc2 and 16 times on top
> of 6.0-rc2 + this series. The results for kvm/x86_64 are:
>
>                       v6.0-rc2  Optimized       Difference  Z-score
> Random dense bitmap         ns         ns        ns      %
> find_next_bit:          787735     670546    117189   14.9     3.97
> find_next_zero_bit:     777492     664208    113284   14.6    10.51
> find_last_bit:          830925     687573    143352   17.3     2.35
> find_first_bit:        3874366    3306635    567731   14.7     1.84
> find_first_and_bit:   40677125   37739887   2937238    7.2     1.36
> find_next_and_bit:      347865     304456     43409   12.5     1.35
>
> Random sparse bitmap
> find_next_bit:           19816      14021      5795   29.2     6.10
> find_next_zero_bit:    1318901    1223794     95107    7.2     1.41
> find_last_bit:           14573      13514      1059    7.3     6.92
> find_first_bit:        1313321    1249024     64297    4.9     1.53
> find_first_and_bit:       8921       8098       823    9.2     4.56
> find_next_and_bit:        9796       7176      2620   26.7     5.39
>
> Where the statistics is significant (z-score > 3), the improvement
> is ~15%.
>
> According to the bloat-o-meter, the Image size is 10-11K less:
>
> x86_64/defconfig:
> add/remove: 32/14 grow/shrink: 61/782 up/down: 6344/-16521 (-10177)
>
> arm64/defconfig:
> add/remove: 3/2 grow/shrink: 50/714 up/down: 608/-11556 (-10948)
>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Yury Norov <yury.norov@...il.com>

Reviewed-by: Valentin Schneider <vschneid@...hat.com>