OOM Killer 簡短筆記

緣由

滑 hacker news 時被 Out-of-memory victim selection with BPF 砸到，忽然有點好奇 OOM Killer Source Code。

Source Code

https://elixir.bootlin.com/linux/v6.6/source/mm/oom_kill.c#L192

/**
 * oom_badness - heuristic function to determine which candidate task to kill
 * @p: task struct of which task we should calculate
 * @totalpages: total present RAM allowed for page allocation
 *
 * The heuristic for determining which task to kill is made to be as simple and
 * predictable as possible.  The goal is to return the highest value for the
 * task consuming the most memory to avoid subsequent oom failures.
 */
long oom_badness(struct task_struct *p, unsigned long totalpages)
{
    long points;
    long adj;

    if (oom_unkillable_task(p))
        return LONG_MIN;

    p = find_lock_task_mm(p);
    if (!p)
        return LONG_MIN;

    /*
     * Do not even consider tasks which are explicitly marked oom
     * unkillable or have been already oom reaped or the are in
     * the middle of vfork
     */
    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;
    }

    /*
     * The baseline for the badness score is the proportion of RAM that each
     * task's rss, pagetable and swap space use.
     */
    points = get_mm_rss(p->mm) + get_mm_counter(p->mm, MM_SWAPENTS) +
        mm_pgtables_bytes(p->mm) / PAGE_SIZE;
    task_unlock(p);

    /* Normalize to oom_score_adj units */
    adj *= totalpages / 1000;
    points += adj;

    return points;
}

首先會排除以下 process：

• kthread

• 正在 vfork

• 已經被 oom kill

• 被標記 oom skip

然後根據佔用記憶體（rss, swap, page）計算一個分數，越大越需要被 oom kill。

References

How does the OOM killer decide which process to kill first?

Out-of-memory victim selection with BPF

https://elixir.bootlin.com/linux/v6.6/source/mm/oom_kill.c#L192