Re: [RFC -v2] panic_on_oom_timeout

From: Michal Hocko
Date: Wed Jun 17 2015 - 11:42:14 EST

On Wed 17-06-15 22:59:54, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > But you have a point that we could have
> > - constrained OOM which elevates oom_victims
> > - global OOM killer strikes but wouldn't start the timer
> >
> > This is certainly possible and timer_pending(&panic_on_oom) replacing
> > oom_victims check should help here. I will think about this some more.
> Yes, please.

Fixed in my local version. I will post the new version of the patch
after we settle with the approach.

> > The important thing is to decide what is the reasonable way forward. We
> > have two two implementations of panic based timeout. So we should decide
> > - Should we add a panic timeout at all?
> > - Should be the timeout bound to panic_on_oom?
> > - Should we care about constrained OOM contexts?
> > - If yes should they use the same timeout?
> > - If no should each memcg be able to define its own timeout?
> >
> Exactly.
> > My thinking is that it should be bound to panic_on_oom=1 only until we
> > hear from somebody actually asking for a constrained oom and even then
> > do not allow for too large configuration space (e.g. no per-memcg
> > timeout) or have separate mempolicy vs. memcg timeouts.
> My implementation comes from providing debugging hints when analyzing
> vmcore of a stalled system. I'm posting logs of stalls after global OOM
> killer struck because it is easy to reproduce. But what I have problem
> is when a system stalled before the OOM killer strikes (I saw many cases
> for customer's enterprise servers), for we don't have hints for guessing
> whether memory allocator is the cause or not. Thus, my version tried to
> emit warning messages using sysctl_memalloc_task_warn_secs .

I can understand your frustration but I believe that a debugability is
a separate topic and we should start by defining a reasonable _policy_
so that an administrator has a way to handle potential OOM stalls
reasonably and with a well defined semantic.

> Ability to take care of constrained OOM contexts is a side effect of use of
> per a "struct task_struct" variable. Even if we come to a conclusion that
> we should not add a timeout for panic, I hope that a timeout for warning
> about memory allocation stalls is added.
> > Let's start simple and make things more complicated later!
> I think we mismatch about what the timeout counters are for.

Michal Hocko
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