Re: [patch 2/2] memcg: do not sleep on OOM waitqueue with fullcharge context

From: Michal Hocko
Date: Thu Jun 13 2013 - 09:48:36 EST

On Wed 12-06-13 13:49:47, David Rientjes wrote:
> On Wed, 12 Jun 2013, Michal Hocko wrote:
> > The patch is a big improvement with a minimum code overhead. Blocking
> > any task which sits on top of an unpredictable amount of locks is just
> > broken. So regardless how many users are affected we should merge it and
> > backport to stable trees. The problem is there since ever. We seem to
> > be surprisingly lucky to not hit this more often.
> >
> Right now it appears that that number of users is 0 and we're talking
> about a problem that was reported in 3.2 that was released a year and a
> half ago. The rules of inclusion in stable also prohibit such a change
> from being backported, specifically "It must fix a real bug that bothers
> people (not a, "This could be a problem..." type thing)".

As you can see there is an user seeing this in 3.2. The bug is _real_ and
I do not see what you are objecting against. Do you really think that
sitting on a time bomb is preferred more?

> We have deployed memcg on a very large number of machines and I can run a
> query over all software watchdog timeouts that have occurred by
> deadlocking on i_mutex during memcg oom. It returns 0 results.

Do you capture /prc/<pid>/stack for each of them to find that your
deadlock (and you have reported that they happen) was in fact caused by
a locking issue? These kind of deadlocks might got unnoticed especially
when the oom is handled by userspace by increasing the limit (my mmecg
is stuck and increasing the limit a bit always helped).

> > I am not quite sure I understand your reservation about the patch to be
> > honest. Andrew still hasn't merged this one although 1/2 is in.
> Perhaps he is as unconvinced? The patch adds 100 lines of code, including
> fields to task_struct for memcg, for a problem that nobody can reproduce.
> My question still stands: can anybody, even with an instrumented kernel to
> make it more probable, reproduce the issue this is addressing?

So the referenced discussion is not sufficient?

Michal Hocko
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