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SubjectRe: Improving OOM killer
On Mon, 1 Feb 2010, Lubos Lunak wrote:

> Hello,
>
> I'd like to suggest some changes to the OOM killer code that I believe
> improve its behaviour - here it turns it from something completely useless and
> harmful into something that works very well. I'm first posting changes for
> review, I'll reformat the patch later as needed if approved. Given that it's
> been like this for about a decade, I get the feeling there must be some
> strange catch.
>

Thanks for your detailed explanation! It will be very helpful to have
your perspective and enthusiasm as we address oom killer issues.

I don't quite understand how you can say the oom killer is "completely
useless and harmful." It certainly fulfills its purpose, which is to kill
a memory hogging task so that a page allocation may succeed when reclaim
has failed to free any memory. It's useful in that regard, although I
don't think you'll find any argument that killing user tasks could always
be described as "harmful" in some way, at least to somebody :)

> My scenario is working in a KDE desktop session and accidentally running
> parallel make in doc/ subdirectory of sources of a KDE module. As I use
> distributed compiling, I run make with -j20 or more, but, as the tool used for
> processing KDE documentation is quite memory-intensive, running this many
> of them is more than enough to consume all the 2GB RAM in the machine. What
> happens in that case is that the machine becomes virtually unresponsible,
> where even Ctrl+Alt+F1 can take minutes, not to mention some action that'd
> actually redeem the situation. If I wait long enough for something to happen,
> which can be even hours, the action that ends the situation is killing one of
> the most vital KDE processes, rendering the whole session useless and making
> me lose all unsaved data.
>

That's the point at which direct reclaim can no longer free any memory and
we must invoke the oom killer as opposed to potentially deadlocking the
system.

> The process tree looks roughly like this:
>
> init
> |- kdeinit
> | |- ksmserver
> | | |- kwin
> | |- <other>
> |- konsole
> |- make
> |- sh
> | |- meinproc4
> |- sh
> | |- meinproc4
> |- <etc>
>
> What happens is that OOM killer usually selects either ksmserver (KDE session
> manager) or kdeinit (KDE master process that spawns most KDE processes). Note
> that in either case OOM killer does not reach the point of killing the actual
> offender - it will randomly kill in the tree under kdeinit until it decides
> to kill ksmserver, which means terminating the desktop session. As konsole is
> a KUniqueApplication, it forks into background and gets reparented to init,
> thus getting away from the kdeinit subtree. Since the memory pressure is
> distributed among several meinproc4 processes, the badness does not get
> summed up in its make grandparent, as badness() does this only for direct
> parents.
>

There's no randomness involved in selecting a task to kill; the oom killer
does, however, try to kill a child with seperate memory first instead of
the parent and that selection depends on the ordering of the selected
task's child list. As I mentioned very early this morning, that could
certainly be improved to kill the child with the highest badness() score
first. Something like this untested patch could do it:

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -439,6 +439,9 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
const char *message)
{
struct task_struct *c;
+ struct task_struct *victim = NULL;
+ unsigned long victim_points = 0;
+ struct timespec uptime;

if (printk_ratelimit())
dump_header(p, gfp_mask, order, mem);
@@ -455,14 +458,20 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
printk(KERN_ERR "%s: kill process %d (%s) score %li or a child\n",
message, task_pid_nr(p), p->comm, points);

- /* Try to kill a child first */
+ /* Try to kill the most memory-hogging child first */
+ do_posix_clock_monotonic_gettime(&uptime);
list_for_each_entry(c, &p->children, sibling) {
+ unsigned long cpoints;
+
if (c->mm == p->mm)
continue;
- if (!oom_kill_task(c))
- return 0;
+ cpoints = badness(c, uptime.tv_sec);
+ if (cpoints > victim_points) {
+ victim = c;
+ victim_points = cpoints;
+ }
}
- return oom_kill_task(p);
+ return oom_kill_task(victim ? victim : p);
}

#ifdef CONFIG_CGROUP_MEM_RES_CTLR
This works because the conditions on which oom_kill_task(victim) fails is
when !victim->mm or victim->signal->oom_adj == OOM_DISABLE which we're
guarded against since badness(victim) returns 0 for both.

The process tree that you posted shows a textbook case for using
/proc/pid/oom_adj to ensure a critical task, such as kdeinit is to you, is
protected from getting selected for oom kill. In your own words, this
"spawns most KDE processes," so it's an ideal place to set an oom_adj
value of OOM_DISABLE since that value is inheritable to children and,
thus, all children are implicitly protected as well.

> Each meinproc4 process still uses a considerable amount of memory, so one
> could assume that the situation would be solved by simply killing them one by
> one, but it is not so because of using what I consider poor metric for
> measuring memory usage - VmSize. VmSize, if I'm not mistaken, is the size of
> the address space taken by the process, which in practice does not say much
> about how much memory the process actually uses. For example, /proc/*/status
> for one selected KDE process:
>
> VmPeak: 534676 kB
> VmSize: 528340 kB
> VmLck: 0 kB
> VmHWM: 73464 kB
> VmRSS: 73388 kB
> VmData: 142332 kB
> VmStk: 92 kB
> VmExe: 44 kB
> VmLib: 91232 kB
> VmPTE: 716 kB
>
> And various excerpts from /proc/*/smaps for this process:
> ...
> 7f7b3f800000-7f7b40000000 rwxp 00000000 00:00 0
> Size: 8192 kB
> Rss: 16 kB
> Referenced: 16 kB
> ...
> 7f7b40055000-7f7b44000000 ---p 00000000 00:00 0
> Size: 65196 kB
> Rss: 0 kB
> Referenced: 0 kB
> ...
> 7f7b443cd000-7f7b445cd000 ---p 0001c000 08:01
> 790267 /usr/lib64/kde4/libnsplugin.so
> Size: 2048 kB
> Rss: 0 kB
> Referenced: 0 kB
> ...
> 7f7b48300000-7f7b4927d000 rw-s 00000000 08:01
> 58690 /var/tmp/kdecache-seli/kpc/kde-icon-cache.data
> Size: 15860 kB
> Rss: 24 kB
> Referenced: 24 kB
>
> I assume the first one is stack, search me what the second and third ones are
> (there appears to be one such mapping as the third one for each .so used),
> the last one is a mapping of a large cache file that's nevertheless rarely
> used extensively and even then it's backed by a file. In other words, none of
> this actually uses much of real memory, yet right now it's the process that
> would get killed for using about 70MB memory, even though it's not the
> offender. The offender scores only about 1/3 of its badness, even though it
> uses almost the double amount of memory:
>
> VmPeak: 266508 kB
> VmSize: 266504 kB
> VmLck: 0 kB
> VmHWM: 118208 kB
> VmRSS: 118208 kB
> VmData: 98512 kB
> VmStk: 84 kB
> VmExe: 60 kB
> VmLib: 48944 kB
> VmPTE: 536 kB
>

Using VmSize, however, allows us to define the most important task to kill
for the oom killer: memory leakers. Memory leakers are the single most
important tasks to identify with the oom killer and aren't obvious when
using rss because leaked memory does not stay resident in RAM. I
understand your system may not have such a leaker and it is simply
overcommitted on a 2GB machine, but using rss loses that ability. It also
makes tuning oom killer priorities with /proc/pid/oom_adj almost
impossible since a task's rss is highly dynamic and we cannot speculate on
the state of the VM at the time of oom.

> Therefore, I suggest doing the following changes in mm/oom_kill.c :
>
> =====
> --- linux-2.6.31/mm/oom_kill.c.sav 2010-02-01 22:00:41.614838540 +0100
> +++ linux-2.6.31/mm/oom_kill.c 2010-02-01 22:01:08.773757932 +0100
> @@ -69,7 +69,7 @@ unsigned long badness(struct task_struct
> /*
> * The memory size of the process is the basis for the badness.
> */
> - points = mm->total_vm;
> + points = get_mm_rss(mm);
>
> /*
> * After this unlock we can no longer dereference local variable `mm'
> @@ -83,21 +83,6 @@ unsigned long badness(struct task_struct
> return ULONG_MAX;
>
> /*
> - * Processes which fork a lot of child processes are likely
> - * a good choice. We add half the vmsize of the children if they
> - * have an own mm. This prevents forking servers to flood the
> - * machine with an endless amount of children. In case a single
> - * child is eating the vast majority of memory, adding only half
> - * to the parents will make the child our kill candidate of choice.
> - */
> - list_for_each_entry(child, &p->children, sibling) {
> - task_lock(child);
> - if (child->mm != mm && child->mm)
> - points += child->mm->total_vm/2 + 1;
> - task_unlock(child);
> - }
> -
> - /*
> * 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.
> =====
>
> In other words, use VmRSS for measuring memory usage instead of VmSize, and
> remove child accumulating.
>
> I hope the above is good enough reason for the first change. VmSize includes
> things like read-only mappings, memory mappings that is actually unused,
> mappings backed by a file, mappings from video drivers, and so on. VmRSS is
> actual real memory used, which is what mostly matters here. While it may not
> be perfect, it is certainly an improvement.
>

It's not for a large number of users, the consumer of the largest amount
of rss is not necessarily the task we always want to kill. Just because
an order-0 page allocation fails does not mean we want to kill the task
that would free the largest amount of RAM.

I understand that KDE is extremely important to your work environment and
if you lose it, it seems like a failure of Linux and the VM. However, the
kernel cannot possibly know what applications you believe to be the most
important. For that reason, userspace is able to tune the badness() score
by writing to /proc/pid/oom_adj as I've suggested you do for kdeinit. You
have the ability to protect KDE from getting oom killed, you just need to
use it.

The natural response to this is: well, you can't possibly expect all users
to tune /proc/pid/oom_adj just in case we are oom. I can expect KDE to
protect itself, though, since it is of sufficient importance to its users
that it should protect itself, as I expect the same of the other examples
that have arisen in recent months concerning Xorg.

There's a second caveat, however, to just protecting applications from the
oom killer using OOM_DISABLE. And that goes back to my original point of
being able to define memory leakers. /proc/pid/oom_adj actually works
quite well to define when a task is using far more memory than expected.
You can say the VmSize doesn't accurately reflect the amount of memory an
application is using, but it is both static enough and well understood
from a pure arithemtic standpoint for what a sane range is for a
particular application. It is then possible to use oom_adj to define when
that memory usage is egregious and considered "rogue" but the oom killer
so that it warrants being killed.

> The second change should be done on the basis that it does more harm than
> good. In this specific case, it does not help to identify the source of the
> problem, and it incorrectly identifies kdeinit as the problem solely on the
> basis that it spawned many other processes. I think it's already quite hinted
> that this is a problem by the fact that you had to add a special protection
> for init - any session manager, process launcher or even xterm used for
> launching apps is yet another init.
>

The kernel can't be expected to know that, you must tell it via
/proc/pid/oom_adj.

> I also have problems finding a case where the child accounting would actually
> help. I mean, in practice, I can certainly come up with something in theory,
> and this looks to me like a solution to a very synthesized problem. In which
> realistic case will one process launch a limited number of children, where
> all of them will consume memory, but just killing the children one by one
> won't avoid the problem reasonably? This is unlikely to avoid a forkbomb, as
> in that case the number of children will be the problem. It is not necessary
> for just one children misbehaving and being restarted, nor will it work
> there. So what is that supposed to fix, and is it more likely than the case
> of a process launching several unrelated children?
>

Right, I believe Kame is working on a forkbomb detector that would replace
this logic.

> Moreover, the remedy is almost instant, there is only a fraction of second of
> when the machine is overloaded by the I/O of swapping pages in and out (I do
> not use swap, but there is a large amount of memory used by read-only
> mappings of binaries, libraries or various other files that is in the
> original case rendering the machine unresponsive - I assume this is because
> the kernel tries to kill an innocent process, but the offenders immediatelly
> consume anything that is freed, requiring even memory used by code that is to
> be executed to be swapped in from files again).
>

We may need to look at block congestion timeouts before invoking the oom
killer in the page allocator.


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