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SubjectRe: [External] Re: [PATCH v3] mm/vmscan: add a fatal signals check in drop_slab_node
On Tue, Sep 15, 2020 at 8:13 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 15-09-20 19:40:01, zangchunxin@bytedance.com wrote:
> > 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 | |
>
> I am not sure this is very helpful for this patch but whatever.
>

Yes, it looks a bit messy, I will delete that.

> > We need one path to stop the process.
>
> I would use the following instead
> "
> Add a bail out on the fatal signals in the main loop so that the
> operation can be terminated by userspace.
> "
>

Thanks, will do that :)

> >
> > Signed-off-by: Chunxin Zang <zangchunxin@bytedance.com>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> Acked-by: Michal Hocko <mhocko@suse.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
> >
>
> --
> Michal Hocko
> SUSE Labs

Best wishes
Chunxin

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