Re: [PATCH 2/3] find: micro-optimize for_each_{set,clear}_bit()

Yury Norov
2021-06-19T17:28:14Z

On Sat, Jun 19, 2021 at 05:24:15PM +0100, Marc Zyngier wrote:
> On Fri, 18 Jun 2021 20:57:34 +0100,
> Yury Norov <yury.norov@gmail.com> wrote:
> >
> > The macros iterate thru all set/clear bits in a bitmap. They search a
> > first bit using find_first_bit(), and the rest bits using find_next_bit().
> >
> > Since find_next_bit() is called shortly after find_first_bit(), we can
> > save few lines of I-cache by not using find_first_bit().
>
> Really?
>
> >
> > Signed-off-by: Yury Norov <yury.norov@gmail.com>
> > ---
> > include/linux/find.h | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/find.h b/include/linux/find.h
> > index 4500e8ab93e2..ae9ed52b52b8 100644
> > --- a/include/linux/find.h
> > +++ b/include/linux/find.h
> > @@ -280,7 +280,7 @@ unsigned long find_next_bit_le(const void *addr, unsigned
> > #endif
> >
> > #define for_each_set_bit(bit, addr, size) \
> > - for ((bit) = find_first_bit((addr), (size)); \
> > + for ((bit) = find_next_bit((addr), (size), 0); \
>
> On which architecture do you observe a gain? Only 32bit ARM and m68k
> implement their own version of find_first_bit(), and everyone else
> uses the canonical implementation:

And those who enable GENERIC_FIND_FIRST_BIT - x86, arm64, arc, mips
and s390.

> #ifndef find_first_bit
> #define find_first_bit(addr, size) find_next_bit((addr), (size), 0)
> #endif
>
> These architectures explicitly have different implementations for
> find_first_bit() and find_next_bit() because they can do better
> (whether that is true or not is another debate). I don't think you
> should remove this optimisation until it has been measured on these
> two architectures.

This patch is based on a series that enables separate implementation
of find_first_bit() for all architectures; according to my tests,
find_first* is ~ twice faster than find_next* on arm64 and x86.

https://lore.kernel.org/lkml/20210612123639.329047-1-yury.norov@gmail.com/T/#t

After applying the series, I noticed that my small kernel module that
calls for_each_set_bit() is now using find_first_bit() to just find
one bit, and find_next_bit() for all others. I think it's better to
always use find_next_bit() in this case to minimize the chance of
cache miss. But if it's not that obvious, I'll try to write some test.