Re: Linux 5.19-rc8

Russell King (Oracle)
2022-08-01T15:48:35Z

Oh FFS.

I see you decided off your own back to remove the ARM version of the
find_bit functions, with NO agreement from the arch maintainer. This
is not on.

On Sat, Jul 30, 2022 at 02:38:38PM -0700, Yury Norov wrote:
> On Wed, Jul 27, 2022 at 08:43:22AM +0100, Russell King (Oracle) wrote:
> > On Tue, Jul 26, 2022 at 06:33:55PM -0700, Yury Norov wrote:
> > > On Tue, Jul 26, 2022 at 5:15 PM Russell King (Oracle)
> > > <linux@armlinux.org.uk> wrote:
> > > >
> > > > 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.
> > >
> > > Look at find_{first,next}_and_bit results. Those two have no arch version
> > > and in both cases use generic code. In theory they should be equally fast
> > > before and after, but your testing says that generic case is slower even
> > > for them, and the difference is comparable with real arch functions numbers.
> > > It makes me feel like:
> > > - there's something unrelated, like governor/throttling that affect results;
> > > - the numbers are identical, taking the dispersion into account.
> > >
> > > If the difference really concerns you, I'd suggest running the test
> > > several times
> > > to measure confidence intervals.
> >
> > Given that the benchmark is run against random bitmaps and with
> > interrupts enabled, there is going to be noise in the results.
> >
> > Here's the second run:
> >
> > [26234.429389]
> > Start testing find_bit() with random-filled bitmap
> > [26234.439722] find_next_bit: 2206687 ns, 164277 iterations
> > [26234.448664] find_next_zero_bit: 2188368 ns, 163404 iterations
> > [26234.457612] find_last_bit: 2223742 ns, 164278 iterations
> > [26234.482056] find_first_bit: 17720726 ns, 16384 iterations
> > [26234.859374] find_first_and_bit: 370602019 ns, 32877 iterations
> > [26234.867379] find_next_and_bit: 1280651 ns, 74091 iterations
> > [26234.874107]
> > Start testing find_bit() with sparse bitmap
> > [26234.881014] find_next_bit: 46142 ns, 656 iterations
> > [26234.891900] find_next_zero_bit: 4158987 ns, 327025 iterations
> > [26234.898672] find_last_bit: 49727 ns, 656 iterations
> > [26234.912504] find_first_bit: 7107862 ns, 656 iterations
> > [26234.919290] find_first_and_bit: 52092 ns, 1 iterations
> > [26234.926076] find_next_and_bit: 60856 ns, 1 iterations
> >
> > And a third run:
> >
> > [26459.679524]
> > Start testing find_bit() with random-filled bitmap
> > [26459.689871] find_next_bit: 2199418 ns, 163311 iterations
> > [26459.698798] find_next_zero_bit: 2181289 ns, 164370 iterations
> > [26459.707738] find_last_bit: 2213638 ns, 163311 iterations
> > [26459.732224] find_first_bit: 17764152 ns, 16429 iterations
> > [26460.133823] find_first_and_bit: 394886375 ns, 32672 iterations
> > [26460.141818] find_next_and_bit: 1269693 ns, 73485 iterations
> > [26460.148545]
> > Start testing find_bit() with sparse bitmap
> > [26460.155433] find_next_bit: 40753 ns, 653 iterations
> > [26460.166307] find_next_zero_bit: 4148211 ns, 327028 iterations
> > [26460.173078] find_last_bit: 50017 ns, 653 iterations
> > [26460.187007] find_first_bit: 7205325 ns, 653 iterations
> > [26460.193790] find_first_and_bit: 49358 ns, 1 iterations
> > [26460.200577] find_next_and_bit: 62332 ns, 1 iterations
> >
> > My gut feeling is that yes, there is some variance, but not on an
> > order that is significant that would allow us to say "there's no
> > difference".
> >
> > find_next_bit results for random are: 2222703, 2206687, 2199418,
> > which is an average of 2209603 and a variance of around 0.5%.
> > The difference between this and the single generic figure I have
> > is on the order of 20%.
> >
> > I'll do the same with find_first_bit for random: 17722203, 17720726,
> > and 17764152. Average is 17735694. Variance is around 0.1% or 0.2%.
> > The difference between this and the single generic figure I have is
> > on the order of 5%. Not so large, but still quite a big difference
> > compared to the variance.
> >
> > find_first_bit for sparse: 7562371, 7107862, 7205325. Average is
> > 7291853. Variance is higher at about 4%. Difference between this and
> > the generic figure is 0.5%, so this one is not significantly
> > different.
> >
> > The best result looks to be find_next_zero_bit for the sparse bitmap
> > case. The generic code measures 5.5ms, the native code is sitting
> > around 4.1ms. That's a difference of around 34%, and by just looking
> > at the range in the figures above we can see this is a significant
> > result without needing to do the calculations. Similar is true of
> > find_next_bit for the sparse bitmap.
> >
> > So, I think the results are significant in most cases and variance
> > doesn't account for the differences. The only one which isn't is
> > find_first_bit for the sparse case.
>
> Hi Russel,
>
> + Alexey Klimov <klimov.linux@gmail.com>
>
> This is my testings for native vs generic find_bit operations on a15
> and 17.
>
> The raw numbers are collected by Alexey Klimov on Odroid-xu3. All cpu
> frequencies were fixed at 1000Mhz. (Thanks a lot!)
>
> For each native/generic @ a15/a7 configuration, the find_bit_benchmark
> was run 5 times, and the results are summarized below:
>
> A15 Native Generic Difference
> Dense ns ns % sigmas
> find_next_bit: 3746929 3231641 14 8.3
> find_next_zero_bit: 3935354 3202608 19 10.4
> find_last_bit: 3134713 3129717 0 0.1
> find_first_bit: 85626542 20498669 76 172.4
> find_first_and_bit: 409252997 414820417 -1 -0.2
> find_next_and_bit: 1678706 1654420 1 0.4
>
> Sparse
> find_next_bit: 143208 77924 46 29.4
> find_next_zero_bit: 6893375 6059177 12 14.3
> find_last_bit: 67174 68616 -2 -1.0
> find_first_bit: 33689256 8151493 76 47.8
> find_first_and_bit: 124758 156974 -26 -1.3
> find_next_and_bit: 53391 56716 -6 -0.2
>
> A7 Native Generic Difference
> Dense ns ns % sigmas
> find_next_bit: 4207627 5532764 -31 -14.9
> find_next_zero_bit: 4259961 5236880 -23 -10.0
> find_last_bit: 4281386 4201025 2 1.5
> find_first_bit: 236913620 50970424 78 163.3
> find_first_and_bit: 728069762 750580781 -3 -0.7
> find_next_and_bit: 2696263 2766077 -3 -0.9
>
> Sparse
> find_next_bit: 327241 143776 56 40.7
> find_next_zero_bit: 6987249 10235989 -46 -21.8
> find_last_bit: 97758 94725 3 0.6
> find_first_bit: 94628040 21051964 78 41.8
> find_first_and_bit: 248133 241267 3 0.3
> find_next_and_bit: 136475 154000 -13 -0.5
>
> The last column is the difference between native and generic code
> performance normalized to a standard deviation:
> (mean(native) - mean(generic)) / max(std(native), std(generic))
>
> The results look consistent to me because 'and' subtests that are always
> generic differ by less than one sigma.
>
> On A15 generic code is a clear winner. On A7 results are inconsistent
> although significant. Maybe it's worth to retest on A7.
>
> Regarding the choice between native and generic core - I would prefer
> generic version even if it's slightly slower because it is tested and
> maintained better. And because the results of the test are at least on
> par, to me it's a no-brainer.
>
> Would be really interesting to compare performance of your LDRB->LDR
> patch with the generic code using the same procedure.
>
> Thanks,
> Yury
>

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