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Subject[PATCH] mm: make fault_around_bytes configurable
Date
Mapping pages around fault is found to cause performance degradation
in certain use cases. The test performed here is launch of 10 apps
one by one, doing something with the app each time, and then repeating
the same sequence once more, on an ARM 64-bit Android device with 2GB
of RAM. The time taken to launch the apps is found to be better when
fault around feature is disabled by setting fault_around_bytes to page
size (4096 in this case).

The tests were done on 3.18 kernel. 4 extra vmstat counters were added
for debugging. pgpgoutclean accounts the clean pages reclaimed via
__delete_from_page_cache. pageref_activate, pageref_activate_vm_exec,
and pageref_keep accounts the mapped file pages activated and retained
by page_check_references.

=== Without swap ===
3.18 3.18-fault_around_bytes=4096
-----------------------------------------------------------------------
workingset_refault 691100 664339
workingset_activate 210379 179139
pgpgin 4676096 4492780
pgpgout 163967 96711
pgpgoutclean 1090664 990659
pgalloc_dma 3463111 3328299
pgfree 3502365 3363866
pgactivate 568134 238570
pgdeactivate 752260 392138
pageref_activate 315078 121705
pageref_activate_vm_exec 162940 55815
pageref_keep 141354 51011
pgmajfault 24863 23633
pgrefill_dma 1116370 544042
pgscan_kswapd_dma 1735186 1234622
pgsteal_kswapd_dma 1121769 1005725
pgscan_direct_dma 12966 1090
pgsteal_direct_dma 6209 967
slabs_scanned 1539849 977351
pageoutrun 1260 1333
allocstall 47 7

=== With swap ===
3.18 3.18-fault_around_bytes=4096
-----------------------------------------------------------------------
workingset_refault 597687 878109
workingset_activate 167169 254037
pgpgin 4035424 5157348
pgpgout 162151 85231
pgpgoutclean 928587 1225029
pswpin 46033 17100
pswpout 237952 127686
pgalloc_dma 3305034 3542614
pgfree 3354989 3592132
pgactivate 626468 355275
pgdeactivate 990205 771902
pageref_activate 294780 157106
pageref_activate_vm_exec 141722 63469
pageref_keep 121931 63028
pgmajfault 67818 45643
pgrefill_dma 1324023 977192
pgscan_kswapd_dma 1825267 1720322
pgsteal_kswapd_dma 1181882 1365500
pgscan_direct_dma 41957 9622
pgsteal_direct_dma 25136 6759
slabs_scanned 689575 542705
pageoutrun 1234 1538
allocstall 110 26

Looks like with fault_around, there is more pressure on reclaim because
of the presence of more mapped pages, resulting in more IO activity,
more faults, more swapping, and allocstalls.

Make fault_around_bytes configurable so that it can be tuned to avoid
performance degradation.

Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
---
mm/Kconfig | 10 ++++++++++
mm/memory.c | 2 +-
2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index f644106..e3476fd 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -681,6 +681,16 @@ config ZONE_DEVICE

If FS_DAX is enabled, then say Y.

+config FAULT_AROUND_BYTES
+ int
+ range 4096 65536
+ default 65536
+ help
+ The number of bytes to be mapped around the fault. The default
+ value of 64 kilobytes effectively disables faultaround on
+ architectures with page size >= 64k, considering the fact that
+ the feature is less relevant when page size is bigger than 4k.
+
config FRAME_VECTOR
bool

diff --git a/mm/memory.c b/mm/memory.c
index 758b0b4..be06714 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2939,7 +2939,7 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
}

static unsigned long fault_around_bytes __read_mostly =
- rounddown_pow_of_two(65536);
+ rounddown_pow_of_two(CONFIG_FAULT_AROUND_BYTES);

#ifdef CONFIG_DEBUG_FS
static int fault_around_bytes_get(void *data, u64 *val)
--
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