linux-kernel - Re: [PATCH] mm/vmscan.c: drop_slab

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Message-ID: <YGwMD3DOymOFJ7O5@dhcp22.suse.cz>
Date:   Tue, 6 Apr 2021 09:21:51 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Neil Sun <neilsun@...ify.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmscan.c: drop_slab_node with task's memcg

On Fri 02-04-21 18:38:54, Neil Sun wrote:
> This patch makes shrink_slab() with task's memcg in drop_slab_node(),
> so we can free reclaimable slab objects belongs to memcg /lxc/i-vbe1u8o7
> with following command:

You are changing semantic of the existing user interface. This knob has
never been memcg aware and it is supposed to have a global impact. I do
not think we can simply change that without some users being surprised
or even breaking them.

> cgexec -g memory:/lxc/i-vbe1u8o7 sysctl vm.drop_caches=2
> 
> Test with following steps:
> 
> root@...l0pwrt8:~# free -h
>               total        used        free      shared  buff/cache   available
> Mem:           62Gi       265Mi        62Gi       1.0Mi       290Mi        61Gi
> Swap:          31Gi          0B        31Gi
> root@...l0pwrt8:~# (cd /tmp && /root/generate_slab_cache)
> root@...l0pwrt8:~# free -h
>               total        used        free      shared  buff/cache   available
> Mem:           62Gi       266Mi        60Gi       1.0Mi       2.2Gi        61Gi
> Swap:          31Gi          0B        31Gi
> root@...l0pwrt8:~# cgcreate -g memory:/lxc/i-vbe1u8o7
> root@...l0pwrt8:~# cgexec -g memory:/lxc/i-vbe1u8o7 /root/generate_slab_cache
> root@...l0pwrt8:~# free -h
>               total        used        free      shared  buff/cache   available
> Mem:           62Gi       267Mi        58Gi       1.0Mi       4.1Gi        61Gi
> Swap:          31Gi          0B        31Gi
> root@...l0pwrt8:~# cgexec -g memory:/lxc/i-vbe1u8o7 sysctl vm.drop_caches=2
> vm.drop_caches = 2
> root@...l0pwrt8:~# free -h
>               total        used        free      shared  buff/cache   available
> Mem:           62Gi       268Mi        60Gi       1.0Mi       2.2Gi        61Gi
> Swap:          31Gi          0B        31Gi
> root@...l0pwrt8:~# sysctl vm.drop_caches=2
> vm.drop_caches = 2
> root@...l0pwrt8:~# free -h
>               total        used        free      shared  buff/cache   available
> Mem:           62Gi       267Mi        62Gi       1.0Mi       290Mi        61Gi
> Swap:          31Gi          0B        31Gi
> 
> Signed-off-by: Neil Sun <neilsun@...ify.com>
> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 562e87cb..81d770a 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -702,7 +702,7 @@ void drop_slab_node(int nid)
>  			return;
>  
>  		freed = 0;
> -		memcg = mem_cgroup_iter(NULL, NULL, NULL);
> +		memcg = mem_cgroup_from_task(current);
>  		do {
>  			freed += shrink_slab(GFP_KERNEL, nid, memcg, 0);
>  		} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs