Subject: Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

From: Mel Gorman <[email-masked]>
Date: Sat, 7 Mar 2015 17:37:20 +0000

On Sat, Mar 07, 2015 at 05:36:58PM +0100, Ingo Molnar wrote:
> 
> * Mel Gorman <[email-masked]> 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?
> 

Elapsed time is primarily worse on one benchmark -- numa01 which is an
adverse workload. The user time differences are also dominated by that
benchmark

                                           4.0.0-rc1             4.0.0-rc1                3.19.0
                                             vanilla         slowscan-v2r7               vanilla
Time User-NUMA01                  32883.59 (  0.00%)    35288.00 ( -7.31%)    25695.96 ( 21.86%)
Time User-NUMA01_THEADLOCAL       17453.20 (  0.00%)    17765.79 ( -1.79%)    17404.36 (  0.28%)
Time User-NUMA02                   2063.70 (  0.00%)     2063.22 (  0.02%)     2037.65 (  1.26%)
Time User-NUMA02_SMT                983.70 (  0.00%)      976.01 (  0.78%)      981.02 (  0.27%)

> > 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.
> 

Yes.

> 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?
> 

The patch makes NUMA hinting faults use standard page table handling routines
and protections to trap the faults. Fundamentally it's safer even though
it appears to cause more traps to be handled. I've been assuming this is
related to the different permissions PTEs get and when they are visible on
all CPUs. This path is addressing the symptom that more faults are being
handled and that it needs to be less aggressive.

I've gone through that patch and didn't spot anything else that is doing
wrong that is not already handled in this series. Did you spot anything
obviously wrong in that patch that isn't addressed in this series?

> 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?
> 

Because it also means reverting all the PROT_NONE handling and going back
to _PAGE_NUMA tricks which I expect would be naked by Linus.

-- 
Mel Gorman
SUSE Labs