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From:   Chunxin Zang <zangchunxin@bytedance.com>
Date:   Tue, 15 Sep 2020 21:04:54 +0800
Message-ID: <CAKRVAeOwV7EBupU9dRbGJvj_M=jS=8WF+B_fEb6B7M2oN1_8TA@mail.gmail.com>
Subject: Re: [External] Re: [PATCH v3] mm/vmscan: add a fatal signals check in drop_slab_node
To:     Michal Hocko <mhocko@suse.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Muchun Song <songmuchun@bytedance.com>
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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