lkml.org 
[lkml]   [2022]   [Sep]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 3/4] lib/find_bit: optimize find_next_bit() functions
    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

    \
     
     \ /
      Last update: 2022-09-20 03:45    [W:3.029 / U:0.048 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site