lowmemorykiller

简称lmk，是安卓平台特有的特性，代码路径在drivers/staging/android/lowmemorykiller.c (kernel 3.18)，sysfs node在/sys/module/lowmemorykiller/下，它的前身是linux OOM，主要两个参数是minfree和adj，minfree的设置主要根据framework层代码算出，但也有厂商会自己修改，要做充分的UX测试后给出。

主体扫描

static unsigned long lowmem_scan(struct shrinker *s, struct shrink_control *sc)
{
    struct task_struct *tsk;
    struct task_struct *selected = NULL;
    unsigned long rem = 0;
    int tasksize;
    int i;
    short min_score_adj = OOM_SCORE_ADJ_MAX + 1;
    int selected_tasksize = 0;
    short selected_oom_score_adj;
    int array_size = ARRAY_SIZE(lowmem_adj);
    int other_free = global_page_state(NR_FREE_PAGES) - totalreserve_pages;
    int other_file = global_page_state(NR_FILE_PAGES) -
                        global_page_state(NR_SHMEM) -
                        total_swapcache_pages();

    if (lowmem_adj_size < array_size)
        array_size = lowmem_adj_size;
    if (lowmem_minfree_size < array_size)
        array_size = lowmem_minfree_size;
    for (i = 0; i < array_size; i++) {
        if (other_free < lowmem_minfree[i] &&
            other_file < lowmem_minfree[i]) {
            min_score_adj = lowmem_adj[i];
            break;
        }
    }

    lowmem_print(3, "lowmem_scan %lu, %x, ofree %d %d, ma %hd\n",
            sc->nr_to_scan, sc->gfp_mask, other_free,
            other_file, min_score_adj);

    if (min_score_adj == OOM_SCORE_ADJ_MAX + 1) {
        lowmem_print(5, "lowmem_scan %lu, %x, return 0\n",
                 sc->nr_to_scan, sc->gfp_mask);
        return 0;
    }

    selected_oom_score_adj = min_score_adj;
        ...
}

首先看看能不能找到oom_score_adj，如果找不到就认为内存充足不杀进程，如果找到就按一定的策略选出某个进程杀掉。

判断内存是否充足的条件就是other_free和other_file两个都必须同时小于lowmem_minfree中的用户设定值,other_free基本上是free pages，other_file基本上是file pages，两者可以分别看成proc/meminfo的MemFree和Cached大小，一般来说Cached远大于MemFree。

static unsigned long lowmem_scan(struct shrinker *s, struct shrink_control *sc)
{
        ...
    rcu_read_lock();
    for_each_process(tsk) {
        struct task_struct *p;
        short oom_score_adj;

        if (tsk->flags & PF_KTHREAD)
            continue;

        p = find_lock_task_mm(tsk);
        if (!p)
            continue;

        if (test_tsk_thread_flag(p, TIF_MEMDIE) &&
            time_before_eq(jiffies, lowmem_deathpending_timeout)) {
            task_unlock(p);
            rcu_read_unlock();
            return 0;
        }
        oom_score_adj = p->signal->oom_score_adj;
        if (oom_score_adj < min_score_adj) {
            task_unlock(p);
            continue;
        }
        tasksize = get_mm_rss(p->mm);
        task_unlock(p);
        if (tasksize <= 0)
            continue;
        if (selected) {
            if (oom_score_adj < selected_oom_score_adj)
                continue;
            if (oom_score_adj == selected_oom_score_adj &&
                tasksize <= selected_tasksize)
                continue;
        }
        selected = p;
        selected_tasksize = tasksize;
        selected_oom_score_adj = oom_score_adj;
        lowmem_print(2, "select %d (%s), adj %hd, size %d, to kill\n",
                 p->pid, p->comm, oom_score_adj, tasksize);
    }
    if (selected) {
        lowmem_print(1, "send sigkill to %d (%s), adj %hd, size %d\n",
                 selected->pid, selected->comm,
                 selected_oom_score_adj, selected_tasksize);
        lowmem_deathpending_timeout = jiffies + HZ;
        set_tsk_thread_flag(selected, TIF_MEMDIE);
        send_sig(SIGKILL, selected, 0);
        rem += selected_tasksize;
    }

    lowmem_print(4, "lowmem_scan %lu, %x, return %lu\n",
             sc->nr_to_scan, sc->gfp_mask, rem);
    rcu_read_unlock();
    return rem;
}

遍历所有的进程for_each_process，当前任务如果是PF_KTHREAD就不能杀，如果不是用find_lock_task_mm遍历当前进程子线程，找到持有->mm的子线程。

如果该线程有TIF_MEMDIE标记就不杀了，如果没有看下该线程的oom_score_adj要比min_score_adj小就直接continue到下一个进程如果比较大而且rss有值(>0)就作为备选后再查看下一个进程。

下一个进程必须比上一次选中的得分要大 or 得分相同但是rss比上一次的占用多的就被认为是更搓的task，要杀。

lmk是如何被调用的

[<c082a824>] (lowmem_scan) from [<c01e0ba4>] (shrink_slab_node+0x204/0x3d0)
[<c01e0ba4>] (shrink_slab_node) from [<c01e12b8>] (shrink_slab+0x70/0xe4)
[<c01e12b8>] (shrink_slab) from [<c01e3ba8>] (try_to_free_pages+0x3c0/0x74c)
[<c01e3ba8>] (try_to_free_pages) from [<c01d9188>] (__alloc_pages_nodemask+0x578/0x92c)
static struct shrinker lowmem_shrinker = {
    .scan_objects = lowmem_scan,
    .count_objects = lowmem_count,
    .seeks = DEFAULT_SEEKS * 16
};

主要是回收内存流程时会调用，lowmem_scan挂到了register_shrinker里, shrink_slab_node里会scan_objects。