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Message-ID: <YykaRYstXwJGqwvB@yury-laptop>
Date: Mon, 19 Sep 2022 18:41:25 -0700
From: Yury Norov <yury.norov@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.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 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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