linux-kernel - Re: [PATCH v1] mm:memcg: skip memcg of current in mem_cgroup_soft_limit

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Message-ID: <20180803061527.GA27245@dhcp22.suse.cz>
Date:   Fri, 3 Aug 2018 08:15:27 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-patch-test@...ts.linaro.org
Subject: Re: [PATCH v1] mm:memcg: skip memcg of current in
 mem_cgroup_soft_limit_reclaim

On Fri 03-08-18 13:48:05, Zhaoyang Huang wrote:
> for the soft_limit reclaim has more directivity than global reclaim, we
> have current memcg be skipped to avoid potential page thrashing.

a) this changelog doesn't really explain the problem nor does it explain
   why the proposed solution is reasonable or why it works at all and
b) no, this doesn't really work. You could easily break the current soft
   limit semantic.

I understand that you are not really happy about how the soft limit
works. Me neither but this whole interface is a huge mistake of past and
the general recommendation is to not use it. We simply cannot fix it
because it is unfixable. The semantic is just broken and somebody might
really depend on it.

> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...eadtrum.com>
> ---
>  mm/memcontrol.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8c0280b..9d09e95 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2537,12 +2537,21 @@ unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order,
>  			mz = mem_cgroup_largest_soft_limit_node(mctz);
>  		if (!mz)
>  			break;
> -
> +		/*
> +		 * skip current memcg to avoid page thrashing, for the
> +		 * mem_cgroup_soft_reclaim has more directivity than
> +		 * global reclaim.
> +		 */
> +		if (get_mem_cgroup_from_mm(current->mm) == mz->memcg) {
> +			reclaimed = 0;
> +			goto next;
> +		}
>  		nr_scanned = 0;
>  		reclaimed = mem_cgroup_soft_reclaim(mz->memcg, pgdat,
>  						    gfp_mask, &nr_scanned);
>  		nr_reclaimed += reclaimed;
>  		*total_scanned += nr_scanned;
> +next:
>  		spin_lock_irq(&mctz->lock);
>  		__mem_cgroup_remove_exceeded(mz, mctz);
>  
> -- 
> 1.9.1

-- 
Michal Hocko
SUSE Labs