* Mel Gorman [off-list ref] wrote:

Dave Chinner reported the following on https://lkml.org/lkml/2015/3/1/226

Across the board the 4.0-rc1 numbers are much slower, and the 
degradation is far worse when using the large memory footprint 
configs. Perf points straight at the cause - this is from 4.0-rc1 on 
the "-o bhash=101073" config:

[...]

           4.0.0-rc1   4.0.0-rc1      3.19.0
             vanilla  slowscan-v2     vanilla
User        53384.29    56093.11    46119.12
System        692.14      311.64      306.41
Elapsed      1236.87     1328.61     1039.88

Note that the system CPU usage is now similar to 3.19-vanilla.

Similar, but still worse, and also the elapsed time is still much 
worse. User time is much higher, although it's the same amount of work 
done on every kernel, right?

I also tested with a workload very similar to Dave's. The machine 
configuration and storage is completely different so it's not an 
equivalent test unfortunately. It's reporting the elapsed time and 
CPU time while fsmark is running to create the inodes and when 
runnig xfsrepair afterwards

xfsrepair
                                    4.0.0-rc1             4.0.0-rc1                3.19.0
                                      vanilla           slowscan-v2               vanilla
Min      real-fsmark        1157.41 (  0.00%)     1150.38 (  0.61%)     1164.44 ( -0.61%)
Min      syst-fsmark        3998.06 (  0.00%)     3988.42 (  0.24%)     4016.12 ( -0.45%)
Min      real-xfsrepair      497.64 (  0.00%)      456.87 (  8.19%)      442.64 ( 11.05%)
Min      syst-xfsrepair      500.61 (  0.00%)      263.41 ( 47.38%)      194.97 ( 61.05%)
Amean    real-fsmark        1166.63 (  0.00%)     1155.97 (  0.91%)     1166.28 (  0.03%)
Amean    syst-fsmark        4020.94 (  0.00%)     4004.19 (  0.42%)     4025.87 ( -0.12%)
Amean    real-xfsrepair      507.85 (  0.00%)      459.58 (  9.50%)      447.66 ( 11.85%)
Amean    syst-xfsrepair      519.88 (  0.00%)      281.63 ( 45.83%)      202.93 ( 60.97%)
Stddev   real-fsmark           6.55 (  0.00%)        3.97 ( 39.30%)        1.44 ( 77.98%)
Stddev   syst-fsmark          16.22 (  0.00%)       15.09 (  6.96%)        9.76 ( 39.86%)
Stddev   real-xfsrepair       11.17 (  0.00%)        3.41 ( 69.43%)        5.57 ( 50.17%)
Stddev   syst-xfsrepair       13.98 (  0.00%)       19.94 (-42.60%)        5.69 ( 59.31%)
CoeffVar real-fsmark           0.56 (  0.00%)        0.34 ( 38.74%)        0.12 ( 77.97%)
CoeffVar syst-fsmark           0.40 (  0.00%)        0.38 (  6.57%)        0.24 ( 39.93%)
CoeffVar real-xfsrepair        2.20 (  0.00%)        0.74 ( 66.22%)        1.24 ( 43.47%)
CoeffVar syst-xfsrepair        2.69 (  0.00%)        7.08 (-163.23%)        2.80 ( -4.23%)
Max      real-fsmark        1171.98 (  0.00%)     1159.25 (  1.09%)     1167.96 (  0.34%)
Max      syst-fsmark        4033.84 (  0.00%)     4024.53 (  0.23%)     4039.20 ( -0.13%)
Max      real-xfsrepair      523.40 (  0.00%)      464.40 ( 11.27%)      455.42 ( 12.99%)
Max      syst-xfsrepair      533.37 (  0.00%)      309.38 ( 42.00%)      207.94 ( 61.01%)

The key point is that system CPU usage for xfsrepair (syst-xfsrepair)
is almost cut in half. It's still not as low as 3.19-vanilla but it's
much closer

                             4.0.0-rc1   4.0.0-rc1      3.19.0
                               vanilla  slowscan-v2     vanilla
NUMA alloc hit               146138883   121929782   104019526
NUMA alloc miss               13146328    11456356     7806370
NUMA interleave hit                  0           0           0
NUMA alloc local             146060848   121865921   103953085
NUMA base PTE updates        242201535   117237258   216624143
NUMA huge PMD updates           113270       52121      127782
NUMA page range updates      300195775   143923210   282048527
NUMA hint faults             180388025    87299060   147235021
NUMA hint local faults        72784532    32939258    61866265
NUMA hint local percent             40          37          42
NUMA pages migrated           71175262    41395302    23237799

Note the big differences in faults trapped and pages migrated. 
3.19-vanilla still migrated fewer pages but if necessary the 
threshold at which we start throttling migrations can be lowered.

This too is still worse than what v3.19 had.

So what worries me is that Dave bisected the regression to:

  4d9424669946 ("mm: convert p[te|md]_mknonnuma and remaining page table manipulations")

And clearly your patch #4 just tunes balancing/migration intensity - 
is that a workaround for the real problem/bug?

And the patch Dave bisected to is a relatively simple patch.
Why not simply revert it to see whether that cures much of the 
problem?

Am I missing something fundamental?

Thanks,

	Ingo