[PATCH 1/1] mm: thp: Redefine default THP defrag behaviour disable it by default

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This patch only makes sense on mmotm because it's heavily relying on an
existing swapping-related fix and indirectly relying on the kcompactd
patches. Even though the kernel says "4.4.0", the swapping and kcompactd
patches have been cherry-picked from mmotm for the purposes of testing.

THP defrag is enabled by default to direct reclaim/compact but not wake
kswapd in the event of a THP allocation failure. The problem is that THP
allocation requests potentially enter reclaim/compaction. This potentially
incurs a severe stall that is not guaranteed to be offset by reduced TLB
misses. While there has been considerable effort to reduce the impact
of reclaim/compaction, it is still a high cost and workloads that should
fit in memory fail to do so. Specifically, a simple anon/file streaming
workload will enter direct reclaim on NUMA at least even though the working
set size is 80% of RAM. It's been years and it's time to throw in the towel.

First, this patch redefines what THP defrag means;

o GFP_TRANSHUGE by default will neither reclaim/compact nor wake kswapd
o For faults, defrag will not direct/reclaim but will wake kswapd
o For khugepaged, defrag will enter direct/reclaim but not wake kswapd

This means that a THP fault will no longer stall but may incur
reclaim/compaction via kswapd reclaiming and kcompactd compacting. This
is potentially destructive so the patch disables THP defrag by default.
THP defrag for khugepaged remains enabled and will enter direct/reclaim
but no wakeup kswapd or kcompactd.

After this patch a THP allocation failure will quickly fallback and rely
on khugepaged to recover the situation at some time in the future. In
some cases, this will reduce THP usage but the benefit of THP is hard to
measure and not a universal win where as a stall to reclaim/compaction is
definitely measurable and can be painful.

The first test for this is using "usemem" to read a large file and write
a large anonymous mapping (to avoid the zero page) multiple times. The
total size of the mappings is 80% of RAM and the benchmark simply measures
how long it takes to complete. It uses multiple threads to see if that
is a factor. On UMA, the performance is almost identical so is not reported
but on NUMA, we see this

usemem
                                   4.4.0                 4.4.0
                          kcompactd-v1r1         nodefrag-v1r3
Amean    System-1       102.86 (  0.00%)       46.81 ( 54.50%)
Amean    System-4        37.85 (  0.00%)       34.02 ( 10.12%)
Amean    System-7        48.12 (  0.00%)       46.89 (  2.56%)
Amean    System-12       51.98 (  0.00%)       56.96 ( -9.57%)
Amean    System-21       80.16 (  0.00%)       79.05 (  1.39%)
Amean    System-30      110.71 (  0.00%)      107.17 (  3.20%)
Amean    System-48      127.98 (  0.00%)      124.83 (  2.46%)
Amean    Elapsd-1       185.84 (  0.00%)      105.51 ( 43.23%)
Amean    Elapsd-4        26.19 (  0.00%)       25.58 (  2.33%)
Amean    Elapsd-7        21.65 (  0.00%)       21.62 (  0.16%)
Amean    Elapsd-12       18.58 (  0.00%)       17.94 (  3.43%)
Amean    Elapsd-21       17.53 (  0.00%)       16.60 (  5.33%)
Amean    Elapsd-30       17.45 (  0.00%)       17.13 (  1.84%)
Amean    Elapsd-48       15.40 (  0.00%)       15.27 (  0.82%)

For a single thread, the benchmark completes 43.23% faster with
this patch applied with smaller benefits as the thread increases.
Similar, notice the large reduction in most cases in system CPU
usage. The overall CPU time is

               4.4.0       4.4.0
        kcompactd-v1r1 nodefrag-v1r3
User        10357.65    10438.33
System       3988.88     3543.94
Elapsed      2203.01     1634.41

Which is substantial. Now, the reclaim figures

                                 4.4.0       4.4.0
                          kcompactd-v1r1nodefrag-v1r3
Minor Faults                 128458477   278352931
Major Faults                   2174976         225
Swap Ins                      16904701           0
Swap Outs                     17359627           0
Allocation stalls                43611           0
DMA allocs                           0           0
DMA32 allocs                  19832646    19448017
Normal allocs                614488453   580941839
Movable allocs                       0           0
Direct pages scanned          24163800           0
Kswapd pages scanned                 0           0
Kswapd pages reclaimed               0           0
Direct pages reclaimed        20691346           0
Compaction stalls                42263           0
Compaction success                 938           0
Compaction failures              41325           0

This patch eliminates almost all swapping and direct reclaim activity. There
is still overhead but it's from NUMA balancing which does not identify that
it's pointless trying to do anything with this workload.

I also tried the thpscale benchmark which forces a corner case where compaction
can be used heavily and measures the latency of whether base or huge pages were
used

thpscale Fault Latencies
                                       4.4.0                 4.4.0
                              kcompactd-v1r1         nodefrag-v1r3
Amean    fault-base-1      5288.84 (  0.00%)     2817.12 ( 46.73%)
Amean    fault-base-3      6365.53 (  0.00%)     3499.11 ( 45.03%)
Amean    fault-base-5      6526.19 (  0.00%)     4363.06 ( 33.15%)
Amean    fault-base-7      7142.25 (  0.00%)     4858.08 ( 31.98%)
Amean    fault-base-12    13827.64 (  0.00%)    10292.11 ( 25.57%)
Amean    fault-base-18    18235.07 (  0.00%)    13788.84 ( 24.38%)
Amean    fault-base-24    21597.80 (  0.00%)    24388.03 (-12.92%)
Amean    fault-base-30    26754.15 (  0.00%)    19700.55 ( 26.36%)
Amean    fault-base-32    26784.94 (  0.00%)    19513.57 ( 27.15%)
Amean    fault-huge-1      4223.96 (  0.00%)     2178.57 ( 48.42%)
Amean    fault-huge-3      2194.77 (  0.00%)     2149.74 (  2.05%)
Amean    fault-huge-5      2569.60 (  0.00%)     2346.95 (  8.66%)
Amean    fault-huge-7      3612.69 (  0.00%)     2997.70 ( 17.02%)
Amean    fault-huge-12     3301.75 (  0.00%)     6727.02 (-103.74%)
Amean    fault-huge-18     6696.47 (  0.00%)     6685.72 (  0.16%)
Amean    fault-huge-24     8000.72 (  0.00%)     9311.43 (-16.38%)
Amean    fault-huge-30    13305.55 (  0.00%)     9750.45 ( 26.72%)
Amean    fault-huge-32     9981.71 (  0.00%)    10316.06 ( -3.35%)

The average time to fault pages is substantially reduced in the
majority of caseds but with the obvious caveat that fewer THPs
are actually used in this adverse workload

                                   4.4.0                 4.4.0
                          kcompactd-v1r1         nodefrag-v1r3
Percentage huge-1         0.71 (  0.00%)       14.04 (1865.22%)
Percentage huge-3        10.77 (  0.00%)       33.05 (206.85%)
Percentage huge-5        60.39 (  0.00%)       38.51 (-36.23%)
Percentage huge-7        45.97 (  0.00%)       34.57 (-24.79%)
Percentage huge-12       68.12 (  0.00%)       40.07 (-41.17%)
Percentage huge-18       64.93 (  0.00%)       47.82 (-26.35%)
Percentage huge-24       62.69 (  0.00%)       44.23 (-29.44%)
Percentage huge-30       43.49 (  0.00%)       55.38 ( 27.34%)
Percentage huge-32       50.72 (  0.00%)       51.90 (  2.35%)

                                 4.4.0       4.4.0
                          kcompactd-v1r1nodefrag-v1r3
Minor Faults                  37429143    47564000
Major Faults                      1916        1558
Swap Ins                          1466        1079
Swap Outs                      2936863      149626
Allocation stalls                62510           3
DMA allocs                           0           0
DMA32 allocs                   6566458     6401314
Normal allocs                216361697   216538171
Movable allocs                       0           0
Direct pages scanned          25977580       17998
Kswapd pages scanned                 0     3638931
Kswapd pages reclaimed               0      207236
Direct pages reclaimed         8833714          88
Compaction stalls               103349           5
Compaction success                 270           4
Compaction failures             103079           1

Note again that while this does swap as it's an aggressive workload,
the direct relcim activity and allocation stalls is substantially
reduced. There is some kswapd activity but ftrace showed that the
kswapd activity was due to normal wakeups from 4K pages being
allocated. Compaction-related stalls and activity are almost
eliminated.

I also tried the stutter benchmark. For this, I do not have figures for
NUMA but it's something that does impact UMA so I'll report what is available

stutter
                                 4.4.0                 4.4.0
                        kcompactd-v1r1         nodefrag-v1r3
Min         mmap      7.3571 (  0.00%)      7.3438 (  0.18%)
1st-qrtle   mmap      7.5278 (  0.00%)     17.9200 (-138.05%)
2nd-qrtle   mmap      7.6818 (  0.00%)     21.6055 (-181.25%)
3rd-qrtle   mmap     11.0889 (  0.00%)     21.8881 (-97.39%)
Max-90%     mmap     27.8978 (  0.00%)     22.1632 ( 20.56%)
Max-93%     mmap     28.3202 (  0.00%)     22.3044 ( 21.24%)
Max-95%     mmap     28.5600 (  0.00%)     22.4580 ( 21.37%)
Max-99%     mmap     29.6032 (  0.00%)     25.5216 ( 13.79%)
Max         mmap   4109.7289 (  0.00%)   4813.9832 (-17.14%)
Mean        mmap     12.4474 (  0.00%)     19.3027 (-55.07%)

This benchmark is trying to fault an anonymous mapping while there is
a heavy IO load -- a scenario that desktop users used to complain about
frequently. This shows a mix because the ideal case of mapping with THP
is not hit as often. However, note that 99% of the mappings complete
13.79% faster. The CPU usage here is particularly interesting

               4.4.0       4.4.0
        kcompactd-v1r1nodefrag-v1r3
User           67.50        0.99
System       1327.88       91.30
Elapsed      2079.00     2128.98

And once again we look at the reclaim figures

                                 4.4.0       4.4.0
                          kcompactd-v1r1nodefrag-v1r3
Minor Faults                 335241922  1314582827
Major Faults                       715         819
Swap Ins                             0           0
Swap Outs                            0           0
Allocation stalls               532723           0
DMA allocs                           0           0
DMA32 allocs                1822364341  1177950222
Normal allocs               1815640808  1517844854
Movable allocs                       0           0
Direct pages scanned          21892772           0
Kswapd pages scanned          20015890    41879484
Kswapd pages reclaimed        19961986    41822072
Direct pages reclaimed        21892741           0
Compaction stalls              1065755           0
Compaction success                 514           0
Compaction failures            1065241           0

Allocation stalls and all direct reclaim activity is eliminated as well
as compaction-related stalls.

THP gives impressive gains in some cases but only if they are quickly
available.  We're not going to reach the point where they are completely
free so lets take the costs out of the fast paths finally and defer the
cost to kswapd, kcompactd and khugepaged where it belongs.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
 include/linux/gfp.h |  2 +-
 mm/huge_memory.c    | 26 ++++++++++++++++----------
 2 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 8942af0813e3..e4a0287e5d0b 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -248,7 +248,7 @@ struct vm_area_struct;
 #define GFP_HIGHUSER_MOVABLE	(GFP_HIGHUSER | __GFP_MOVABLE)
 #define GFP_TRANSHUGE	((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
 			 __GFP_NOMEMALLOC | __GFP_NORETRY | __GFP_NOWARN) & \
-			 ~__GFP_KSWAPD_RECLAIM)
+			 ~__GFP_RECLAIM)
 
 /* Convert GFP flags to their corresponding migrate type */
 #define GFP_MOVABLE_MASK (__GFP_RECLAIMABLE|__GFP_MOVABLE)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 62fe06bb7d04..2708e9766e37 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -46,7 +46,6 @@ unsigned long transparent_hugepage_flags __read_mostly =
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE_MADVISE
 	(1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG)|
 #endif
-	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_FLAG)|
 	(1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG)|
 	(1<<TRANSPARENT_HUGEPAGE_USE_ZERO_PAGE_FLAG);
 
@@ -784,9 +783,17 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm,
 	return 0;
 }
 
-static inline gfp_t alloc_hugepage_gfpmask(int defrag, gfp_t extra_gfp)
+/* Defrag for allocation during fault will wake kswapd if necessary */
+static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
 {
-	return (GFP_TRANSHUGE & ~(defrag ? 0 : __GFP_RECLAIM)) | extra_gfp;
+	bool defrag = transparent_hugepage_defrag(vma);
+	return GFP_TRANSHUGE | (defrag ? __GFP_KSWAPD_RECLAIM : 0);
+}
+
+/* Defrag for khugepaged will enter direct reclaim/compaction if necessary */
+static inline gfp_t alloc_hugepage_khugepaged_gfpmask(void)
+{
+	return GFP_TRANSHUGE | (khugepaged_defrag() ? __GFP_DIRECT_RECLAIM : 0);
 }
 
 /* Caller must hold page table lock. */
@@ -859,7 +866,7 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		}
 		return ret;
 	}
-	gfp = alloc_hugepage_gfpmask(transparent_hugepage_defrag(vma), 0);
+	gfp = alloc_hugepage_direct_gfpmask(vma);
 	page = alloc_hugepage_vma(gfp, vma, haddr, HPAGE_PMD_ORDER);
 	if (unlikely(!page)) {
 		count_vm_event(THP_FAULT_FALLBACK);
@@ -1185,7 +1192,7 @@ int do_huge_pmd_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 alloc:
 	if (transparent_hugepage_enabled(vma) &&
 	    !transparent_hugepage_debug_cow()) {
-		huge_gfp = alloc_hugepage_gfpmask(transparent_hugepage_defrag(vma), 0);
+		huge_gfp = alloc_hugepage_direct_gfpmask(vma);
 		new_page = alloc_hugepage_vma(huge_gfp, vma, haddr, HPAGE_PMD_ORDER);
 	} else
 		new_page = NULL;
@@ -2440,9 +2447,9 @@ static int khugepaged_find_target_node(void)
 	return 0;
 }
 
-static inline struct page *alloc_hugepage(int defrag)
+static inline struct page *alloc_khugepaged_hugepage(void)
 {
-	return alloc_pages(alloc_hugepage_gfpmask(defrag, 0),
+	return alloc_pages(alloc_hugepage_khugepaged_gfpmask(),
 			   HPAGE_PMD_ORDER);
 }
 
@@ -2451,7 +2458,7 @@ static struct page *khugepaged_alloc_hugepage(bool *wait)
 	struct page *hpage;
 
 	do {
-		hpage = alloc_hugepage(khugepaged_defrag());
+		hpage = alloc_khugepaged_hugepage();
 		if (!hpage) {
 			count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
 			if (!*wait)
@@ -2523,8 +2530,7 @@ static void collapse_huge_page(struct mm_struct *mm,
 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
 
 	/* Only allocate from the target node */
-	gfp = alloc_hugepage_gfpmask(khugepaged_defrag(), __GFP_OTHER_NODE) |
-		__GFP_THISNODE;
+	gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_OTHER_NODE | __GFP_THISNODE;
 
 	/* release the mmap_sem read lock. */
 	new_page = khugepaged_alloc_page(hpage, gfp, mm, address, node);
-- 
2.6.4

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