On 9/9/20 5:20 PM, zangchunxin@xxxxxxxxxxxxx wrote:
> From: Chunxin Zang <zangchunxin@xxxxxxxxxxxxx>
>
> On our server, there are about 10k memcg in one machine. They use memory
> very frequently. When I tigger drop caches，the process will infinite loop
> in drop_slab_node.
>
> There are two reasons:
> 1.We have too many memcgs, even though one object freed in one memcg, the
> sum of object is bigger than 10.
>
> 2.We spend a lot of time in traverse memcg once. So, the memcg who
> traversed at the first have been freed many objects. Traverse memcg next
> time, the freed count bigger than 10 again.
>
> 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 | |
>
> In the while loop, we can check whether the TASK_KILLABLE signal is set,
> if so, we should break the loop.
That's definitely a good change, thanks. I would just maybe consider:
- Test in the memcg iteration loop? If you have 10k memcgs as you mention, this
can still take long until the test happens?
- Exit also on other signals such as SIGABRT, SIGTERM? If I write to drop_caches
and think it's too long, I would prefer to kill it by ctrl-c and not just kill
-9. Dunno if the canonical way of testing for this is if
(signal_pending(current)) or differently.
- IMHO it's still worth to bail out in your scenario even without a signal, e.g.
by the doubling of threshold. But it can be a separate patch.
Thanks!
> Signed-off-by: Chunxin Zang <zangchunxin@xxxxxxxxxxxxx>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
> changelogs in v2:
> 1) Via check TASK_KILLABLE signal break loop.
>
> mm/vmscan.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b6d84326bdf2..c3ed8b45d264 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 (fatal_signal_pending(current))
> + return;
> +
> freed = 0;
> memcg = mem_cgroup_iter(NULL, NULL, NULL);
> do {
>
