On Fri 26-08-11 09:09:46, Michal Hocko wrote:
> On Thu 25-08-11 14:14:20, David Rientjes wrote:
> > On Thu, 25 Aug 2011, Michal Hocko wrote:
> >
> > > > > > That's obviously false since we call oom_killer_disable() in
> > > > > > freeze_processes() to disable the oom killer from ever being called in the
> > > > > > first place, so this is something you need to resolve with Rafael before
> > > > > > you cause more machines to panic.
> > > > >
> > > > > I didn't mean suspend/resume path (that is protected by oom_killer_disabled)
> > > > > so the patch doesn't make any change.
> > > >
> > > > Confused... freeze_processes() does try_to_freeze_tasks() before
> > > > oom_killer_disable() ?
> > >
> > > Yes you are right, I must have been blind.
> > >
> > > Now I see the point. We do not want to panic while we are suspending and
> > > the memory is really low just because all the userspace is already in
> > > the the fridge.
> > > Sorry for confusion.
> > >
> > > I still do not follow the oom_killer_disable note from David, though.
> > >
> >
> > oom_killer_disable() was added to that path for a reason when all threads
> > are frozen: memory allocations still occur in the suspend path in an oom
> > condition and adding the oom_killer_disable() will cause those
> > allocations to fail rather than sending pointless SIGKILLs to frozen
> > threads.
> >
> > Now consider if the only _eligible_ threads for oom kill (because of
> > cpusets or mempolicies) are those that are frozen. We certainly do not
> > want to panic because other cpusets are still getting work done. We'd
> > either want to add a mem to the cpuset or thaw the processes because the
> > cpuset is oom.
>
> Sure.
>
> >
> > You can't just selectively skip certain threads when their state can be
> > temporary without risking a panic. That's why this patch is a
> > non-starter.
> >
> > A much better solution would be to lower the badness score that the oom
> > killer uses for PF_FROZEN threads so that they aren't considered a
> > priority for kill unless there's nothing else left to kill.
>
> Yes, sounds better.
.. but still not sufficient. We also have to thaw the process
as well. Just a quick hacked up patch (not tested, just for an
illustration).
Would something like this work?
---
>From 305a8139a72b20709e6b59ff8f4d322a9e04ab19 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@xxxxxxx>
Date: Fri, 26 Aug 2011 10:39:35 +0200
Subject: [PATCH] oom: do not live lock on frozen tasks
[WARNING untested]
OOM can end up in a live lock if select_bad_process picks up a frozen
task. On the other hand we cannot mark such processes as unkillable
because we could panic the system even though there is a chance that
somebody could thaw the process so we can make a forward process (e.g.
a process from another cpuset or with a different nodemask).
Let's give all frozen tasks a bonus (OOM_SCORE_ADJ_MAX/2) so that we do
not consider them unless really necessary and if we really pick up one
then thaw its threads before we try to kill it.
TODO
- given bonus might be too big?
- aren't we racing with try_to_freeze_tasks?
---
mm/oom_kill.c | 13 +++++++++++++
1 files changed, 13 insertions(+), 0 deletions(-)
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 626303b..fd194bc 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -32,6 +32,7 @@
#include <linux/mempolicy.h>
#include <linux/security.h>
#include <linux/ptrace.h>
+#include <linux/freezer.h>
int sysctl_panic_on_oom;
int sysctl_oom_kill_allocating_task;
@@ -214,6 +215,14 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
points += p->signal->oom_score_adj;
/*
+ * Do not try to kill frozen tasks unless there is nothing else to kill.
+ * We do not want to give it 1 point because we still want to select a good
+ * candidate among all frozen tasks. Let's give it a reasonable bonus.
+ */
+ if (frozen(p))
+ points -= OOM_SCORE_ADJ_MAX/2;
+
+ /*
* Never return 0 for an eligible task that may be killed since it's
* possible that no single user task uses more than 0.1% of memory and
* no single admin tasks uses more than 3.0%.
@@ -450,6 +459,10 @@ static int oom_kill_task(struct task_struct *p, struct mem_cgroup *mem)
pr_err("Kill process %d (%s) sharing same memory\n",
task_pid_nr(q), q->comm);
task_unlock(q);
+
+ if (frozen(q))
+ thaw_process(q);
+
force_sig(SIGKILL, q);
}
--
1.7.5.4
--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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