Re: Linux 5.19-rc8

[Russell King (Oracle)]

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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxx>
> 
> 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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