Re: Linux 5.19-rc8

Russell King (Oracle)
2022-07-27T00:16:00Z

On Tue, Jul 26, 2022 at 01:20:23PM -0700, Linus Torvalds wrote:
> On Tue, Jul 26, 2022 at 12:44 PM Russell King (Oracle)
> <linux@armlinux.org.uk> wrote:
> >
> > Overall, I would say it's pretty similar (some generic perform
> > marginally better, some native perform marginally better) with the
> > exception of find_first_bit() being much better with the generic
> > implementation, but find_next_zero_bit() being noticably worse.
>
> The generic _find_first_bit() code is actually sane and simple. It
> loops over words until it finds a non-zero one, and then does trivial
> calculations on that last word.
>
> That explains why the generic code does so much better than your byte-wise asm.
>
> In contrast, the generic _find_next_bit() I find almost offensively
> silly - which in turn explains why your byte-wide asm does better.
>
> I think the generic _find_next_bit() should actually do what the m68k
> find_next_bit code does: handle the first special word itself, and
> then just call find_first_bit() on the rest of it.
>
> And it should *not* try to handle the dynamic "bswap and/or bit sense
> invert" thing at all. That should be just four different (trivial)
> cases for the first word.

Here's the results for the native version converted to use word loads:

[ 37.319937]
Start testing find_bit() with random-filled bitmap
[ 37.330289] find_next_bit: 2222703 ns, 163781 iterations
[ 37.339186] find_next_zero_bit: 2154375 ns, 163900 iterations
[ 37.348118] find_last_bit: 2208104 ns, 163780 iterations
[ 37.372564] find_first_bit: 17722203 ns, 16370 iterations
[ 37.737415] find_first_and_bit: 358135191 ns, 32453 iterations
[ 37.745420] find_next_and_bit: 1280537 ns, 73644 iterations
[ 37.752143]
Start testing find_bit() with sparse bitmap
[ 37.759032] find_next_bit: 41256 ns, 655 iterations
[ 37.769905] find_next_zero_bit: 4148410 ns, 327026 iterations
[ 37.776675] find_last_bit: 48742 ns, 655 iterations
[ 37.790961] find_first_bit: 7562371 ns, 655 iterations
[ 37.797743] find_first_and_bit: 47366 ns, 1 iterations
[ 37.804527] find_next_and_bit: 59924 ns, 1 iterations

which is generally faster than the generic version, with the exception
of the sparse find_first_bit (generic was:
[ 25.657304] find_first_bit: 7328573 ns, 656 iterations)

find_next_{,zero_}bit() in the sparse case are quite a bit faster than
the generic code.

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