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

From: Mel Gorman
Date: Sat Mar 07 2015 - 12:37:34 EST

On Sat, Mar 07, 2015 at 05:36:58PM +0100, Ingo Molnar wrote:
>
> * Mel Gorman <mgorman@xxxxxxx> 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
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