[PATCH v3] mm/vmscan: add a fatal signals check in drop_slab_node

[zangchunxin@bytedance.com]

From: Chunxin Zang <zangchunxin@bytedance.com>

On our server, there are about 10k memcg in one machine. They use memory
very frequently. We have observed that drop_caches can take a
considerable amount of time, and can't stop it.

There are two reasons:
1. There is somebody constantly generating more objects to reclaim
   on drop_caches, result the 'freed' always bigger than 10.
2. The process has no chance to process signals.

We can get the following info through 'ps':

  root:~# ps -aux | grep drop
  root  357956 ... R    Aug25 21119854:55 echo 3 > /proc/sys/vm/drop_caches
  root 1771385 ... R    Aug16 21146421:17 echo 3 > /proc/sys/vm/drop_caches
  root 1986319 ... R    18:56 117:27 echo 3 > /proc/sys/vm/drop_caches
  root 2002148 ... R    Aug24 5720:39 echo 3 > /proc/sys/vm/drop_caches
  root 2564666 ... R    18:59 113:58 echo 3 > /proc/sys/vm/drop_caches
  root 2639347 ... R    Sep03 2383:39 echo 3 > /proc/sys/vm/drop_caches
  root 3904747 ... R    03:35 993:31 echo 3 > /proc/sys/vm/drop_caches
  root 4016780 ... R    Aug21 7882:18 echo 3 > /proc/sys/vm/drop_caches

Use bpftrace follow 'freed' value in drop_slab_node:

  root:~# bpftrace -e 'kprobe:drop_slab_node+70 {@ret=hist(reg("bp")); }'
  Attaching 1 probe...
  ^B^C

  @ret:
  [64, 128)        1 |                                                    |
  [128, 256)      28 |                                                    |
  [256, 512)     107 |@                                                   |
  [512, 1K)      298 |@@@                                                 |
  [1K, 2K)       613 |@@@@@@@                                             |
  [2K, 4K)      4435 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
  [4K, 8K)       442 |@@@@@                                               |
  [8K, 16K)      299 |@@@                                                 |
  [16K, 32K)     100 |@                                                   |
  [32K, 64K)     139 |@                                                   |
  [64K, 128K)     56 |                                                    |
  [128K, 256K)    26 |                                                    |
  [256K, 512K)     2 |                                                    |

We need one path to stop the process.

Signed-off-by: Chunxin Zang <zangchunxin@bytedance.com>
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---

	changelogs in v3: 
	1) update the description of the patch.
		v2 named: mm/vmscan: fix infinite loop in drop_slab_node

	changelogs in v2: 
	1) via check fatal signal break loop.

 mm/vmscan.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b6d84326bdf2..6b2b5d420510 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -704,6 +704,9 @@ void drop_slab_node(int nid)
 	do {
 		struct mem_cgroup *memcg = NULL;
 
+		if (signal_pending(current))
+			return;
+
 		freed = 0;
 		memcg = mem_cgroup_iter(NULL, NULL, NULL);
 		do {
-- 
2.11.0