Linux内核OOM killer机制
程序运行了一段时间，有个进程挂掉了，正常情况下进程不会主动挂掉，简单分析后认为可能是运行时某段时间内存占用过大，系统内存不足导致触发了Linux操作系统OOM killer机制，将运行中的进程杀掉了。
一、Linux内核OOM killer机制 Linux 内核有个机制叫OOM killer(Out Of Memory killer)，该机制会监控那些占用内存过大，尤其是瞬间占用内存很快的进程，然后防止内存耗尽而自动把该进程杀掉。内核检测到系统内存不足、挑选并杀掉某个进程的过程可以参考内核源代码linux/mm/oom_kill.c，当系统内存不足的时候，out_of_memory()被触发，然后调用select_bad_process()选择一个”bad”进程杀掉。如何判断和选择一个”bad进程呢？linux选择”bad”进程是通过调用oom_badness()，挑选的算法和想法都很简单很朴实：最bad的那个进程就是那个最占用内存的进程。
【0】、什么时候触发?
内核在触发OOM机制时会调用到out_of_memory()函数，此函数的调用顺序如下：
__alloc_pages //内存分配时调用
|-->__alloc_pages_nodemask
|--> __alloc_pages_slowpath
|--> __alloc_pages_may_oom
| --> out_of_memory //触发
以上函数__alloc_pages_may_oom()在调用之前会先判断oom_killer_disabled的值，如果有值，则不会触发OOM机制；
布尔型变量oom_killer_disabled定义在文件mm/page_alloc.c中，并没有提供外部接口更改此值，但是在内核中此值默认为0，表示打开OOM-kill。
Linux中内存都是以页的形式管理的，所以不管是怎么申请内存，都会调用alloc_page()函数，最终调用到函数out_of_memory()，触发OOM机制。
【1】内核监测到系统内存不足时，该函数被触发执行：
/**
* out_of_memory - kill the "best" process when we run out of memory
* @oc: pointer to struct oom_control
*
* If we run out of memory, we have the choice between either
* killing a random task (bad), letting the system crash (worse)
* OR try to be smart about which process to kill. Note that we
* don't have to be perfect here, we just have to be good.
*/
bool out_of_memory(struct oom_control *oc)
{
unsigned long freed = 0;
enum oom_constraint constraint = CONSTRAINT_NONE;
if (oom_killer_disabled)
return false;
if (!is_memcg_oom(oc)) {
blocking_notifier_call_chain(&oom_notify_list, 0, &freed);
if (freed > 0)
/* Got some memory back in the last second. */
return true;
}
/*
* If current has a pending SIGKILL or is exiting, then automatically
* select it. The goal is to allow it to allocate so that it may
* quickly exit and free its memory.
*/
if (task_will_free_mem(current)) {
mark_oom_victim(current);
wake_oom_reaper(current);
return true;
}
/*
* The OOM killer does not compensate for IO-less reclaim.
* pagefault_out_of_memory lost its gfp context so we have to
* make sure exclude 0 mask - all other users should have at least
* ___GFP_DIRECT_RECLAIM to get here.
*/
if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS))
return true;
/*
* Check if there were limitations on the allocation (only relevant for
* NUMA and memcg) that may require different handling.
*/
constraint = constrained_alloc(oc);
if (constraint != CONSTRAINT_MEMORY_POLICY)
oc->nodemask = NULL;
check_panic_on_oom(oc, constraint);
if (!is_memcg_oom(oc) && sysctl_oom_kill_allocating_task &&
current->mm && !oom_unkillable_task(current, NULL, oc->nodemask) &&
current->signal->oom_score_adj != OOM_SCORE_ADJ_MIN) {
get_task_struct(current);
oc->chosen = current;
oom_kill_process(oc, "Out of memory (oom_kill_allocating_task)");
return true;
}
select_bad_process(oc); //选择一个“最坏的”进程杀掉。
/* Found nothing?!?! */
if (!oc->chosen) {
dump_header(oc, NULL);
pr_warn("Out of memory and no killable processes...\n");
/*
* If we got here due to an actual allocation at the
* system level, we cannot survive this and will enter
* an endless loop in the allocator. Bail out now.
*/
if (!is_sysrq_oom(oc) && !is_memcg_oom(oc))
panic("System is deadlocked on memory\n");
}
if (oc->chosen && oc->chosen != (void *)-1UL)
oom_kill_process(oc, !is_memcg_oom(oc) ? "Out of memory" :
"Memory cgroup out of memory");
return !!oc->chosen;
}
【2】选择一个“最坏的”进程
/*
* Simple selection loop. We choose the process with the highest number of
* 'points'. In case scan was aborted, oc->chosen is set to -1.
*/
static void select_bad_process(struct oom_control *oc)
{
if (is_memcg_oom(oc))
mem_cgroup_scan_tasks(oc->memcg, oom_evaluate_task, oc);
else {
struct task_struct *p;
rcu_read_lock();
for_each_process(p)
if (oom_evaluate_task(p, oc))
break;
rcu_read_unlock();
}
oc->chosen_points = oc->chosen_points * 1000 / oc->totalpages;
}
【3】杀掉进程
static void oom_kill_process(struct oom_control *oc, const char *message)
{
struct task_struct *victim = oc->chosen;
struct mem_cgroup *oom_group;
static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
DEFAULT_RATELIMIT_BURST);
/*
* If the task is already exiting, don't alarm the sysadmin or kill
* its children or threads, just give it access to memory reserves
* so it can die quickly
*/
task_lock(victim);
if (task_will_free_mem(victim)) {
mark_oom_victim(victim);
wake_oom_reaper(victim);
task_unlock(victim);
put_task_struct(victim);
return;
}
task_unlock(victim);
if (__ratelimit(&oom_rs))
dump_header(oc, victim);
/*
* Do we need to kill the entire memory cgroup?
* Or even one of the ancestor memory cgroups?
* Check this out before killing the victim task.
*/
oom_group = mem_cgroup_get_oom_group(victim, oc->memcg);
__oom_kill_process(victim, message);
/*
* If necessary, kill all tasks in the selected memory cgroup.
*/
if (oom_group) {
mem_cgroup_print_oom_group(oom_group);
mem_cgroup_scan_tasks(oom_group, oom_kill_memcg_member,
(void*)message);
mem_cgroup_put(oom_group);
}
}
下面附加两个函数是select_bad_process()函数的实现细节，可不看。
static int oom_evaluate_task(struct task_struct *task, void *arg)
{
struct oom_control *oc = arg;
unsigned long points;
if (oom_unkillable_task(task, NULL, oc->nodemask))
goto next;
/*
* This task already has access to memory reserves and is being killed.
* Don't allow any other task to have access to the reserves unless
* the task has MMF_OOM_SKIP because chances that it would release
* any memory is quite low.
*/
if (!is_sysrq_oom(oc) && tsk_is_oom_victim(task)) {
if (test_bit(MMF_OOM_SKIP, &task->signal->oom_mm->flags))
goto next;
goto abort;
}
/*
* If task is allocating a lot of memory and has been marked to be
* killed first if it triggers an oom, then select it.
*/
if (oom_task_origin(task)) {
points = ULONG_MAX;
goto select;
}
points = oom_badness(task, NULL, oc->nodemask, oc->totalpages);
if (!points || points < oc->chosen_points)
goto next;
/* Prefer thread group leaders for display purposes */
if (points == oc->chosen_points && thread_group_leader(oc->chosen))
goto next;
select:
if (oc->chosen)
put_task_struct(oc->chosen);
get_task_struct(task);
oc->chosen = task;
oc->chosen_points = points;
next:
return 0;
abort:
if (oc->chosen)
put_task_struct(oc->chosen);
oc->chosen = (void *)-1UL;
return 1;
}
/**
* 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
* @memcg: task's memory controller, if constrained
* @nodemask: nodemask passed to page allocator for mempolicy ooms
*
* 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.
*/
unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
const nodemask_t *nodemask, unsigned long totalpages)
{
long points;
long adj;
if (oom_unkillable_task(p, memcg, nodemask))
return 0;
p = find_lock_task_mm(p);
if (!p)
return 0;
/*
* 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 0;
}
/*
* 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;
/*
* Never return 0 for an eligible task regardless of the root bonus and
* oom_score_adj (oom_score_adj can't be OOM_SCORE_ADJ_MIN here).
*/
return points > 0 ? points : 1;
}
二、查看系统日志方法：
运行egrep -i -r 'killed process' /var/log命令，结果如下：
/var/log/syslog.1:May 6 10:02:51 iZuf66b59tpzdaxbchl3d4Z kernel: [1467990.340288] Killed process 17909 (procon) total-vm:5312000kB, anon-rss:4543100kB, file-rss:0kB
也可运行dmesg命令，结果如下：
[1471454.635492] Out of memory: Kill process 17907 (procon) score 143 or sacrifice child
[1471454.636345] Killed process 17907 (procon) total-vm:5617060kB, anon-rss:4848752kB, file-rss:0kB
显示可读时间的话可用dmesg -T查看：
[Wed May 15 14:03:08 2019] Out of memory: Kill process 83446 (machine) score 250 or sacrifice child
[Wed May 15 14:03:08 2019] Killed process 83446 (machine) total-vm:1920560kB, anon-rss:1177488kB, file-rss:1600kB
三、 附录
【1】附加__alloc_pages_nodemask()函数如下：
/*
* This is the 'heart' of the zoned buddy allocator.
*/
struct page *
__alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
nodemask_t *nodemask)
{
struct page *page;
unsigned int alloc_flags = ALLOC_WMARK_LOW;
gfp_t alloc_mask; /* The gfp_t that was actually used for allocation */
struct alloc_context ac = { };
/*
* There are several places where we assume that the order value is sane
* so bail out early if the request is out of bound.
*/
if (unlikely(order >= MAX_ORDER)) {
WARN_ON_ONCE(!(gfp_mask & __GFP_NOWARN));
return NULL;
}
gfp_mask &= gfp_allowed_mask;
alloc_mask = gfp_mask;
if (!prepare_alloc_pages(gfp_mask, order, preferred_nid, nodemask, &ac, &alloc_mask, &alloc_flags))
return NULL;
finalise_ac(gfp_mask, &ac);
/*
* Forbid the first pass from falling back to types that fragment
* memory until all local zones are considered.
*/
alloc_flags |= alloc_flags_nofragment(ac.preferred_zoneref->zone, gfp_mask);
/* First allocation attempt */
page = get_page_from_freelist(alloc_mask, order, alloc_flags, &ac);
if (likely(page))
goto out;
/*
* Apply scoped allocation constraints. This is mainly about GFP_NOFS
* resp. GFP_NOIO which has to be inherited for all allocation requests
* from a particular context which has been marked by
* memalloc_no{fs,io}_{save,restore}.
*/
alloc_mask = current_gfp_context(gfp_mask);
ac.spread_dirty_pages = false;
/*
* Restore the original nodemask if it was potentially replaced with
* &cpuset_current_mems_allowed to optimize the fast-path attempt.
*/
if (unlikely(ac.nodemask != nodemask))
ac.nodemask = nodemask;
page = __alloc_pages_slowpath(alloc_mask, order, &ac);
out:
if (memcg_kmem_enabled() && (gfp_mask & __GFP_ACCOUNT) && page &&
unlikely(__memcg_kmem_charge(page, gfp_mask, order) != 0)) {
__free_pages(page, order);
page = NULL;
}
trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype);
return page;
}
最后，简单分析一下你的进程被Linux杀掉几个可能的原因：一种是内存泄露；一种是你的进程所需要的内存资源太大，系统无法满足，应该在设计时对进程需要的资源有个最大限制，不能让他无限增长；当然，也不一定全是你的问题，也有可能是同一主机的其他进程占用资源过多，但是Linux OOM选择“最坏“进程杀掉的算法是很简单粗暴的，就选中你的进程杀掉，也是有可能的。