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Date:   Tue, 11 Apr 2023 14:23:32 +0200
From:   Michal =?utf-8?Q?Koutn=C3=BD?= <mkoutny@suse.com>
To:     Gang Li <ligang.bdlg@bytedance.com>
Cc:     Waiman Long <longman@redhat.com>, Michal Hocko <mhocko@suse.com>,
        cgroups@vger.kernel.org, linux-mm@kvack.org, rientjes@google.com,
        Zefan Li <lizefan.x@bytedance.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4] mm: oom: introduce cpuset oom
Message-ID: <3myr57cw3qepul7igpifypxx4xd2buo2y453xlqhdw4xgjokc4@vi3odjfo3ahc>
References: <20230411065816.9798-1-ligang.bdlg@bytedance.com>
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Hello.

On Tue, Apr 11, 2023 at 02:58:15PM +0800, Gang Li <ligang.bdlg@bytedance.com> wrote:
> +int cpuset_scan_tasks(int (*fn)(struct task_struct *, void *), void *arg)
> +{
> +	int ret = 0;
> +	struct css_task_iter it;
> +	struct task_struct *task;
> +	struct cpuset *cs;
> +	struct cgroup_subsys_state *pos_css;
> +
> +	/*
> +	 * Situation gets complex with overlapping nodemasks in different cpusets.
> +	 * TODO: Maybe we should calculate the "distance" between different mems_allowed.
> +	 *
> +	 * But for now, let's make it simple. Just iterate through all cpusets
> +	 * with the same mems_allowed as the current cpuset.
> +	 */
> +	cpuset_read_lock();
> +	rcu_read_lock();
> +	cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
> +		if (nodes_equal(cs->mems_allowed, task_cs(current)->mems_allowed)) {
> +			css_task_iter_start(&(cs->css), CSS_TASK_ITER_PROCS, &it);
> +			while (!ret && (task = css_task_iter_next(&it)))
> +				ret = fn(task, arg);
> +			css_task_iter_end(&it);
> +		}
> +	}
> +	rcu_read_unlock();
> +	cpuset_read_unlock();
> +	return ret;
> +}

I see this traverses all cpusets without the hierarchy actually
mattering that much. Wouldn't the CONSTRAINT_CPUSET better achieved by
globally (or per-memcg) scanning all processes and filtering with:
	nodes_intersect(current->mems_allowed, p->mems_allowed)
(`current` triggers the OOM, `p` is the iterated task)
?

Thanks,
Michal

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