From: Gavin Shan gshan@redhat.com Subject: mm/vmscan.c: don't round up scan size for online memory cgroup

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

Link: http://lkml.kernel.org/r/20200211024514.8730-1-gshan@redhat.com Fixes: 68600f623d69 ("mm: don't miss the last page because of round-off error") Signed-off-by: Gavin Shan gshan@redhat.com Acked-by: Roman Gushchin guro@fb.com Cc: stable@vger.kernel.org [4.20+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/vmscan.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/mm/vmscan.c~mm-vmscan-dont-round-up-scan-size-for-online-memory-cgroup +++ a/mm/vmscan.c @@ -2415,10 +2415,13 @@ out: /* * 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: _