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

Yury Norov
2021-02-16T18:01:31Z

On Tue, Feb 16, 2021 at 11:14:23AM +0200, Andy Shevchenko wrote:
> 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.

In this specific case tools follow kernel well.

Nevertheless, if you think it's a blocker for the series, I can split. What
option for tools is better for you - doubling the number of patches or
squashing everything in a patch bomb?