Does setting an oom_score_adj of -1000 actually make a task immune to the kernel's oom_killer?

Environment

• Red Hat Enterprise Linux 7
• Red Hat Enterprise Linux 8
• Red Hat Enterprise Linux 9
• Red Hat OpenShift 4

Issue

• If a task's oom_score_adj value is set to -1000, does this make the task the least likely to be killed, or make it completely immune to the oom_killer completely when it is invoked?

Resolution

• Setting the processes oom_score_adj to -1000 does actually make a process unkillable by the oom_killer when it is invoked.
• Be very careful with setting processes to being immune to the oom_killer,

Root Cause

• When the oom_killer is invoked, the oom_evaluate_task function is invoked and the oom_badness function is checked to get a score for a process's ability to be killed:
  • Content from github.com is not included.https://github.com/torvalds/linux/blob/master/mm/oom_kill.c#L342-L344

static int oom_evaluate_task(struct task_struct *task, void *arg)
[...]
	points = oom_badness(task, oc->totalpages);
	if (points == LONG_MIN || points < oc->chosen_points)
		goto next;

• When the process is evaluated in oom_badness, there are three conditions that exclude a process from being evaluated; it either has a oom_score_adj of OOM_SCORE_ADJ_MIN which correlates to -1000, it is in the middle of a forking operation, or it is flagged as being unkillable (such as is the case with kernel threads):
  • Content from github.com is not included.https://github.com/torvalds/linux/blob/master/mm/oom_kill.c#L219-L225

long oom_badness(struct task_struct *p, unsigned long totalpages)
[...]
	adj = (long)p->signal->oom_score_adj;
	if (adj == OOM_SCORE_ADJ_MIN ||
			test_bit(MMF_OOM_SKIP, &p->mm->flags) ||
			in_vfork(p)) {
		task_unlock(p);
		return LONG_MIN;
	}

• Since -1000 causes this to return LONG_MIN, which is a -1, the oom_evaluate_task function will skip this task from being considered in the list of tasks that can be killed.

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