commit 68600f623d69 ("mm: don't miss the last page because of round-off
error") makes the scan size round up to @denominator regardless of the
memory cgroup's state, online or offline. This affects the overall
reclaiming behavior: The corresponding LRU list is eligible for reclaiming
only when its size logically right shifted by @sc->priority is bigger than
zero in the former formula. For example, the inactive anonymous LRU list
should have at least 0x4000 pages to be eligible for reclaiming when we
have 60/12 for swappiness/priority and without taking scan/rotation ratio
into account. After the roundup is applied, the inactive anonymous LRU
list becomes eligible for reclaiming when its size is bigger than or
equal to 0x1000 in the same condition.
(0x4000 >> 12) * 60 / (60 + 140 + 1) = 1
((0x1000 >> 12) * 60) + 200) / (60 + 140 + 1) = 1
aarch64 has 512MB huge page size when the base page size is 64KB. The
memory cgroup that has a huge page is always eligible for reclaiming
in that case. The reclaiming is likely to stop after the huge page is
reclaimed, meaing the further iteration on @sc->priority and the silbing
and child memory cgroups will be skipped. The overall behaviour has been
changed. This fixes the issue by applying the roundup to offlined memory
cgroups only, to give more preference to reclaim memory from offlined
memory cgroup. It sounds reasonable as those memory is unlikedly to be
used by anyone.
The issue was found by starting up 8 VMs on a Ampere Mustang machine,
which has 8 CPUs and 16 GB memory. Each VM is given with 2 vCPUs and 2GB
memory. It took 264 seconds for all VMs to be completely up and 784MB swap
is consumed after that. With this patch applied, it took 236 seconds and
60MB swap to do same thing. So there is 10% performance improvement for my
case. Note that KSM is disable while THP is enabled in the testing.
total used free shared buff/cache available
Mem: 16196 10065 2049 16 4081 3749
Swap: 8175 784 7391
total used free shared buff/cache available
Mem: 16196 11324 3656 24 1215 2936
Swap: 8175 60 8115
Fixes: 68600f623d69 ("mm: don't miss the last page because of round-off error")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.20+
Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
Acked-by: Roman Gushchin <guro@xxxxxx>
---
v2: Improved commit log to include time used to boot 8 VMs
Include Roman's ACK
---
mm/vmscan.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index c05eb9efec07..876370565455 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2415,10 +2415,13 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
/*
* Scan types proportional to swappiness and
* their relative recent reclaim efficiency.
- * Make sure we don't miss the last page
- * because of a round-off error.
+ * Make sure we don't miss the last page on
+ * the offlined memory cgroups because of a
+ * round-off error.
*/
- scan = DIV64_U64_ROUND_UP(scan * fraction[file],
+ scan = mem_cgroup_online(memcg) ?
+ div64_u64(scan * fraction[file], denominator) :
+ DIV64_U64_ROUND_UP(scan * fraction[file],
denominator);
break;
case SCAN_FILE:
--
2.23.0
