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

From: Mel Gorman
Date: Mon Mar 09 2015 - 17:02:34 EST

On Sun, Mar 08, 2015 at 08:40:25PM +0000, Mel Gorman wrote:
> > Because if the answer is 'yes', then we can safely say: 'we regressed
> > performance because correctness [not dropping dirty bits] comes before
> > performance'.
> >
> > If the answer is 'no', then we still have a mystery (and a regression)
> > to track down.
> >
> > As a second hack (not to be applied), could we change:
> >
> > #define _PAGE_BIT_PROTNONE _PAGE_BIT_GLOBAL
> >
> > to:
> >
> > #define _PAGE_BIT_PROTNONE (_PAGE_BIT_GLOBAL+1)
> >
>
> In itself, that's not enough. The SWP_OFFSET_SHIFT would also need updating
> as a partial revert of 21d9ee3eda7792c45880b2f11bff8e95c9a061fb but it
> can be done.
>

More importantily, _PAGE_BIT_GLOBAL+1 == the special PTE bit so just
updating the value should crash. For the purposes of testing the idea, I
thought the straight-forward option was to break soft dirty page tracking
and steal their bit for testing (patch below). Took most of the day to
get access to the test machine so tests are not long running and only
the autonuma one has completed;

autonumabench
3.19.0 4.0.0-rc1 4.0.0-rc1 4.0.0-rc1
vanilla vanilla slowscan-v2r7 protnone-v3
Time User-NUMA01 25695.96 ( 0.00%) 32883.59 (-27.97%) 35288.00 (-37.33%) 35236.21 (-37.13%)
Time User-NUMA01_THEADLOCAL 17404.36 ( 0.00%) 17453.20 ( -0.28%) 17765.79 ( -2.08%) 17590.10 ( -1.07%)
Time User-NUMA02 2037.65 ( 0.00%) 2063.70 ( -1.28%) 2063.22 ( -1.25%) 2072.95 ( -1.73%)
Time User-NUMA02_SMT 981.02 ( 0.00%) 983.70 ( -0.27%) 976.01 ( 0.51%) 983.42 ( -0.24%)
Time System-NUMA01 194.70 ( 0.00%) 602.44 (-209.42%) 209.42 ( -7.56%) 737.36 (-278.72%)
Time System-NUMA01_THEADLOCAL 98.52 ( 0.00%) 78.10 ( 20.73%) 92.70 ( 5.91%) 80.69 ( 18.10%)
Time System-NUMA02 9.28 ( 0.00%) 6.47 ( 30.28%) 6.06 ( 34.70%) 6.63 ( 28.56%)
Time System-NUMA02_SMT 3.79 ( 0.00%) 5.06 (-33.51%) 3.39 ( 10.55%) 3.60 ( 5.01%)
Time Elapsed-NUMA01 558.84 ( 0.00%) 755.96 (-35.27%) 833.63 (-49.17%) 804.50 (-43.96%)
Time Elapsed-NUMA01_THEADLOCAL 382.54 ( 0.00%) 382.22 ( 0.08%) 395.45 ( -3.37%) 388.12 ( -1.46%)
Time Elapsed-NUMA02 49.83 ( 0.00%) 49.38 ( 0.90%) 50.21 ( -0.76%) 48.99 ( 1.69%)
Time Elapsed-NUMA02_SMT 46.59 ( 0.00%) 47.70 ( -2.38%) 48.55 ( -4.21%) 49.50 ( -6.25%)
Time CPU-NUMA01 4632.00 ( 0.00%) 4429.00 ( 4.38%) 4258.00 ( 8.07%) 4471.00 ( 3.48%)
Time CPU-NUMA01_THEADLOCAL 4575.00 ( 0.00%) 4586.00 ( -0.24%) 4515.00 ( 1.31%) 4552.00 ( 0.50%)
Time CPU-NUMA02 4107.00 ( 0.00%) 4191.00 ( -2.05%) 4120.00 ( -0.32%) 4244.00 ( -3.34%)
Time CPU-NUMA02_SMT 2113.00 ( 0.00%) 2072.00 ( 1.94%) 2017.00 ( 4.54%) 1993.00 ( 5.68%)

3.19.0 4.0.0-rc1 4.0.0-rc1 4.0.0-rc1
vanilla vanillaslowscan-v2r7protnone-v3
User 46119.12 53384.29 56093.11 55882.82
System 306.41 692.14 311.64 828.36
Elapsed 1039.88 1236.87 1328.61 1292.92

So just using a different bit doesn't seem to be it either

3.19.0 4.0.0-rc1 4.0.0-rc1 4.0.0-rc1
vanilla vanillaslowscan-v2r7protnone-v3
NUMA alloc hit 1202922 1437560 1472578 1499274
NUMA alloc miss 0 0 0 0
NUMA interleave hit 0 0 0 0
NUMA alloc local 1200683 1436781 1472226 1498680
NUMA base PTE updates 222840103 304513172 121532313 337431414
NUMA huge PMD updates 434894 594467 237170 658715
NUMA page range updates 445505831 608880276 242963353 674693494
NUMA hint faults 601358 733491 334334 820793
NUMA hint local faults 371571 511530 227171 565003
NUMA hint local percent 61 69 67 68
NUMA pages migrated 7073177 26366701 8607082 31288355

Patch to use a bit other than the global bit for prot none is below.

diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index 8c7c10802e9c..1f243323693c 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -20,16 +20,16 @@
#define _PAGE_BIT_SOFTW2 10 /* " */
#define _PAGE_BIT_SOFTW3 11 /* " */
#define _PAGE_BIT_PAT_LARGE 12 /* On 2MB or 1GB pages */
-#define _PAGE_BIT_SPECIAL _PAGE_BIT_SOFTW1
-#define _PAGE_BIT_CPA_TEST _PAGE_BIT_SOFTW1
+#define _PAGE_BIT_SPECIAL _PAGE_BIT_SOFTW3
+#define _PAGE_BIT_CPA_TEST _PAGE_BIT_SOFTW3
#define _PAGE_BIT_SPLITTING _PAGE_BIT_SOFTW2 /* only valid on a PSE pmd */
-#define _PAGE_BIT_HIDDEN _PAGE_BIT_SOFTW3 /* hidden by kmemcheck */
-#define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW3 /* software dirty tracking */
+#define _PAGE_BIT_HIDDEN _PAGE_BIT_SOFTW1 /* hidden by kmemcheck */
+#define _PAGE_BIT_SOFT_DIRTY _PAGE_BIT_SOFTW1 /* software dirty tracking */
#define _PAGE_BIT_NX 63 /* No execute: only valid after cpuid check */

/* If _PAGE_BIT_PRESENT is clear, we use these: */
/* - if the user mapped it with PROT_NONE; pte_present gives true */
-#define _PAGE_BIT_PROTNONE _PAGE_BIT_GLOBAL
+#define _PAGE_BIT_PROTNONE _PAGE_BIT_SOFTW1

#define _PAGE_PRESENT (_AT(pteval_t, 1) << _PAGE_BIT_PRESENT)
#define _PAGE_RW (_AT(pteval_t, 1) << _PAGE_BIT_RW)
@@ -98,8 +98,7 @@

/* Set of bits not changed in pte_modify */
#define _PAGE_CHG_MASK (PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT | \
- _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY | \
- _PAGE_SOFT_DIRTY)
+ _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY)
#define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)

/*
--
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