Messages in this thread | | | Date | Mon, 19 Sep 2022 18:41:25 -0700 | From | Yury Norov <> | Subject | Re: [PATCH v4 3/4] lib/find_bit: optimize find_next_bit() functions |
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On Mon, Sep 19, 2022 at 04:45:54PM +0300, Andy Shevchenko wrote: > 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.
No, I didn't. I can remove '@' below, if that concerns you.
> > - * 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).
As Linus already said, the 'out' is function-scope. We can make it a block-scope with __label__, but this would make an impression that we are OK with stacking many FIND macros in a single function.
I spend some time trying to figure out a legitimate usecase for it, but nothing came in mind. There are many real cases when we need 2 or more find functions at once but all that cases would work with regular wrappers around FIND_BIT(). Check this, for example:
https://lore.kernel.org/lkml/20220919210559.1509179-6-yury.norov@gmail.com/
I don't know how FIND_BIT() machinery will evolve with time. For now it's a clean and neat local helper with a very straightforward usage. Lets keep it simple now? If someone will decide to call FIND_BIT() twice and fail, it would be a good hint that he's doing something wrong.
> > + 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.
Ok
Thanks, Yury
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