Re: [RESEND PATCH v2 0/6] lib/find_bit: fast path for small bitmaps

Andy Shevchenko
2021-02-16T09:16:56Z

On Mon, Feb 15, 2021 at 01:30:44PM -0800, Yury Norov wrote:
> [add David Laight <David.Laight@ACULAB.COM> ]
>
> On Sat, Jan 30, 2021 at 11:17:11AM -0800, Yury Norov wrote:
> > Bitmap operations are much simpler and faster in case of small bitmaps
> > which fit into a single word. In linux/bitmap.h we have a machinery that
> > allows compiler to replace actual function call with a few instructions
> > if bitmaps passed into the function are small and their size is known at
> > compile time.
> >
> > find_*_bit() API lacks this functionality; despite users will benefit from
> > it a lot. One important example is cpumask subsystem when
> > NR_CPUS <= BITS_PER_LONG. In the very best case, the compiler may replace
> > a find_*_bit() call for such a bitmap with a single ffs or ffz instruction.
> >
> > Tools is synchronized with new implementation where needed.
> >
> > v1: https://www.spinics.net/lists/kernel/msg3804727.html
> > v2: - employ GENMASK() for bitmaps;
> > - unify find_bit inliners in;
> > - address comments to v1;
>
> Comments so far:
> - increased image size (patch #8) - addressed by introducing
> CONFIG_FAST_PATH;

> - split tools and kernel parts - not clear why it's better.

Because tools are user space programs and sometimes may not follow kernel
specifics, so they are different logically and changes should be separated.

> Anything else?

--
With Best Regards,
Andy Shevchenko