Re: [PATCH] fix spurious OOM kills

Thomas Gleixner
2004-11-22T10:59:28Z

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On Sun, 2004-11-21 at 14:57 +0100, Thomas Gleixner wrote:
> On Sun, 2004-11-21 at 13:17 +0100, Martin MOKREJŠ wrote:
> > Why can't the algorithm first find the asking for memory now.
> > When found, kernel should kill first it's children, wait some time,
> > then kill this process if still exists (it might exit itself when children
> > get closed).
> > You have said it's safer to kill that to send ENOMEM as happens
> > in 2.4, but I still don't undertand why kernel first doesn't send
> > ENOMEM, and only if that doesn't help it can start after those 5 seconds
> > OOM killer, and try to kill the very same application.
> > I don't get the idea why to kill immediately.
>
> I see your concern. There are some more changes neccecary to make this
> reliably work. I'm not sure if it can be done without really big
> changes. I will look a bit deeper into this.

One big problem when killing the requesting process or just sending
ENOMEM to the requesting process is, that exactly this process might be
a ssh login, when you try to log into to machine after some application
went crazy and ate up most of the memory. The result is that you
_cannot_ log into the machine, because the login is either killed or
cannot start because it receives ENOMEM.

Putting hard coded decisions like "prefer sshd, xyz,...", " don't kill
a, b, c" are out of discussion.

The ideas which were proposed to have a possibility to set a "don't kill
me" or "yes, I'm a candidate" flag are likely to be a future way to go.
But at the moment we have no way to make this work in current userlands.

I refined the decision, so it does not longer kill the parent, if there
were forked child processes available to kill. So it now should keep
your bash alive.

tglx

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diff -urN --exclude='*~' --exclude='.#*' 2.6.10-rc2-mm2.orig/mm/oom_kill.c 2.6.10-rc2-mm/mm/oom_kill.c
--- 2.6.10-rc2-mm2.orig/mm/oom_kill.c 2004-11-19 14:52:16.000000000 +0100
+++ 2.6.10-rc2-mm/mm/oom_kill.c 2004-11-22 11:53:16.000000000 +0100
@@ -45,8 +45,10 @@
static unsigned long badness(struct task_struct *p, unsigned long uptime)
{
unsigned long points, cpu_time, run_time, s;
+ struct list_head *tsk;

- if (!p->mm)
+ /* Ignore mm-less tasks and init */
+ if (!p->mm || p->pid == 1)
return 0;

if (p->flags & PF_MEMDIE)
@@ -57,6 +59,19 @@
points = p->mm->total_vm;

/*
+ * Processes which fork a lot of child processes are likely
+ * a good choice. We add the vmsize of the childs if they
+ * have an own mm. This prevents forking servers to flood the
+ * machine with an endless amount of childs
+ */
+ list_for_each(tsk, &p->children) {
+ struct task_struct *chld;
+ chld = list_entry(tsk, struct task_struct, sibling);
+ if (chld->mm != p->mm && chld->mm)
+ points += chld->mm->total_vm;
+ }
+
+ /*
* CPU time is in tens of seconds and run time is in thousands
* of seconds. There is no particular reason for this other than
* that it turned out to work very well in practice.
@@ -176,6 +191,27 @@
return mm;
}

+static struct mm_struct *oom_kill_process(task_t *p)
+{
+ struct mm_struct *mm;
+ struct task_struct *g, *q;
+
+ mm = oom_kill_task(p);
+ if (!mm)
+ return NULL;
+ /*
+ * kill all processes that share the ->mm (i.e. all threads),
+ * but are in a different thread group
+ */
+ do_each_thread(g, q)
+ if (q->mm == mm && q->tgid != p->tgid)
+ __oom_kill_task(q);
+
+ while_each_thread(g, q);
+ if (!p->mm)
+ printk(KERN_INFO "Fixed up OOM kill of mm-less task\n");
+ return mm;
+}

/**
* oom_kill - kill the "best" process when we run out of memory
@@ -188,7 +224,9 @@
void oom_kill(void)
{
struct mm_struct *mm;
- struct task_struct *g, *p, *q;
+ struct task_struct *c, *p;
+ struct list_head *tsk;
+ int mmcnt = 0;

read_lock(&tasklist_lock);
retry:
@@ -200,21 +238,32 @@
panic("Out of memory and no killable processes...\n");
}

- mm = oom_kill_task(p);
- if (!mm)
- goto retry;
/*
- * kill all processes that share the ->mm (i.e. all threads),
- * but are in a different thread group
+ * Kill the forked child processes first
*/
- do_each_thread(g, q)
- if (q->mm == mm && q->tgid != p->tgid)
- __oom_kill_task(q);
- while_each_thread(g, q);
- if (!p->mm)
- printk(KERN_INFO "Fixed up OOM kill of mm-less task\n");
+ list_for_each(tsk, &p->children) {
+ c = list_entry(tsk, struct task_struct, sibling);
+ /* Do not touch threads, as they get killed later */
+ if (c->mm == p->mm)
+ continue;
+ mm = oom_kill_process(c);
+ if (mm) {
+ mmcnt ++;
+ mmput(mm);
+ }
+ }
+
+ /*
+ * If we managed to kill one or more child processes
+ * then let the parent live for now
+ */
+ if (!mmcnt) {
+ mm = oom_kill_process(p);
+ if (!mm)
+ goto retry;
+ mmput(mm);
+ }
read_unlock(&tasklist_lock);
- mmput(mm);
return;
}

@@ -224,14 +273,22 @@
void out_of_memory(int gfp_mask)
{
/*
- * oom_lock protects out_of_memory()'s static variables.
- * It's a global lock; this is not performance-critical.
- */
- static DEFINE_SPINLOCK(oom_lock);
+ * inprogress protects out_of_memory()'s static variables.
+ * We don't use a spin_lock here, as spinlocks are
+ * nops on UP systems.
+ */
+ static unsigned long inprogress;
+ static unsigned int freepages = 1000000;
static unsigned long first, last, count, lastkill;
unsigned long now, since;

- spin_lock(&oom_lock);
+ if (test_and_set_bit(0, &inprogress))
+ return;
+
+ /* Check, if memory was freed since the last oom kill */
+ if (freepages < nr_free_pages())
+ goto out_unlock;
+
now = jiffies;
since = now - last;
last = now;
@@ -271,12 +328,10 @@
* Ok, really out of memory. Kill something.
*/
lastkill = now;
-
printk("oom-killer: gfp_mask=0x%x\n", gfp_mask);
show_free_areas();
-
- /* oom_kill() sleeps */
- spin_unlock(&oom_lock);
+ /* Store free pages * 2 for the check above */
+ freepages = (nr_free_pages() << 1);
oom_kill();
/*
* Make kswapd go out of the way, so "p" has a good chance of
@@ -284,17 +339,11 @@
* for more memory.
*/
yield();
- spin_lock(&oom_lock);

reset:
- /*
- * We dropped the lock above, so check to be sure the variable
- * first only ever increases to prevent false OOM's.
- */
- if (time_after(now, first))
- first = now;
+ first = jiffies;
count = 0;

out_unlock:
- spin_unlock(&oom_lock);
+ clear_bit(0, &inprogress);
}