linux-kernel - Re: [v6 3/4] mm, oom: introduce oom

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Date:   Thu, 24 Aug 2017 14:10:54 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, kernel-team@...com,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v6 3/4] mm, oom: introduce oom_priority for memory cgroups

On Wed 23-08-17 17:52:00, Roman Gushchin wrote:
> Introduce a per-memory-cgroup oom_priority setting: an integer number
> within the [-10000, 10000] range, which defines the order in which
> the OOM killer selects victim memory cgroups.

Why do we need a range here?

> OOM killer prefers memory cgroups with larger priority if they are
> populated with eligible tasks.

So this is basically orthogonal to the score based selection and the
real size is only the tiebreaker for same priorities? Could you describe
the usecase? Becasuse to me this sounds like a separate oom killer
strategy. I can imagine somebody might be interested (e.g. always kill
the oldest memcgs...) but an explicit range wouldn't fly with such a
usecase very well.

That brings me back to my original suggestion. Wouldn't a "register an
oom strategy" approach much better than blending things together and
then have to wrap heads around different combinations of tunables?

[...]
> @@ -2760,7 +2761,12 @@ static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
>  			if (iter->oom_score == 0)
>  				continue;
>  
> -			if (iter->oom_score > score) {
> +			if (iter->oom_priority > prio) {
> +				memcg = iter;
> +				prio = iter->oom_priority;
> +				score = iter->oom_score;
> +			} else if (iter->oom_priority == prio &&
> +				   iter->oom_score > score) {
>  				memcg = iter;
>  				score = iter->oom_score;
>  			}

Just a minor thing. Why do we even have to calculate oom_score when we
use it only as a tiebreaker?
-- 
Michal Hocko
SUSE Labs