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Date: Mon, 19 Sep 2022 16:45:54 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
linux-kernel@...r.kernel.org,
Alexey Klimov <klimov.linux@...il.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>,
Valentin Schneider <vschneid@...hat.com>,
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 Wed, Sep 14, 2022 at 07:07:29PM -0700, 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)
...
> /*
Seems like you wanted this to be a kernel doc, but it isn't right now.
> - * This is a common helper function for find_next_bit, find_next_zero_bit, and
> - * find_next_and_bit. The differences are:
> - * - The "invert" argument, which is XORed with each fetched word before
> - * searching it for one bits.
> - * - The optional "addr2", which is anded with "addr1" if present.
> + * Common helper for find_next_bit() function family
In such case this should start with a name of the macro
* FIND_NEXT_BIT - ...
> + * @FETCH: The expression that fetches and pre-processes each word of bitmap(s)
> + * @MUNGE: The expression that post-processes a word containing found bit (may be empty)
> + * @size: The bitmap size in bits
> + * @start: The bitnumber to start searching at
> */
...
> +#define FIND_NEXT_BIT(FETCH, MUNGE, size, start) \
> +({ \
> + unsigned long mask, idx, tmp, sz = (size), __start = (start); \
> + \
> + if (unlikely(__start >= sz)) \
> + goto out; \
> + \
> + mask = MUNGE(BITMAP_FIRST_WORD_MASK(__start)); \
> + idx = __start / BITS_PER_LONG; \
> + \
> + for (tmp = (FETCH) & mask; !tmp; tmp = (FETCH)) { \
> + if ((idx + 1) * BITS_PER_LONG >= sz) \
> + goto out; \
> + idx++; \
> + } \
> + \
> + sz = min(idx * BITS_PER_LONG + __ffs(MUNGE(tmp)), sz); \
> +out: \
I dunno if GCC expression limits the scope of goto labels, but on the safe side
you can add a prefix to it, so it becomes:
FIND_NEXT_BIT_out:
(or alike).
> + sz; \
> +})
...
> +unsigned long _find_next_zero_bit_le(const unsigned long *addr, unsigned
> + long size, unsigned long offset)
Usually we don't split parameters between lines.
...
> +unsigned long _find_next_bit_le(const unsigned long *addr, unsigned
> + long size, unsigned long offset)
Ditto.
--
With Best Regards,
Andy Shevchenko
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