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

Andy Shevchenko
2021-02-17T10:36:23Z

On Tue, Feb 16, 2021 at 10:00:42AM -0800, Yury Norov wrote:
> 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.

It's not a blocker from my side. But you make it harder to push like this,
because you will need a tag from tools, which in my practice is quite
hard to get -> blocker. My point is: don't make obstacles where we can avoid
them. So, if tools won't take this, it won't block us.

> What
> option for tools is better for you - doubling the number of patches or
> squashing everything in a patch bomb?

Not a tools guy, but common sense tells me that the best approach is to follow
kind of changes in the kernel (similar granularity).

--
With Best Regards,
Andy Shevchenko