Re: [PATCH 3/4] OOM, PM: OOM killed task shouldn't escape PM suspend

[Tejun Heo <tj@kernel.org>]

On Thu, Nov 06, 2014 at 05:02:23PM +0100, Michal Hocko wrote:
> > Why would PM freezing make OOM killing fail?  That doesn't make much
> > sense.  Sure, it can block it for a finite duration for sync purposes
> > but making OOM killing fail seems the wrong way around.  
> 
> We cannot block in the allocation path because the request might come
> from the freezer path itself (e.g. when suspending devices etc.).
> At least this is my understanding why the original oom disable approach
> was implemented.

I was saying that it could temporarily block either direction to
implement proper synchronization while guaranteeing forward progress.

> > We're doing one thing for non-PM freezing and the other way around for
> > PM freezing, which indicates one of the two directions is wrong.
> 
> Because those two paths are quite different in their requirements. The
> cgroup freezer only cares about freezing tasks and it doesn't have to
> care about tasks accessing a possibly half suspended device on their way
> out.

I don't think the fundamental relationship between freezing and oom
killing are different between the two and the failure to recognize
that is what's leading to these weird issues.

> > Shouldn't it be that OOM killing happening while PM freezing is in
> > progress cancels PM freezing rather than the other way around?  Find a
> > point in PM suspend/hibernation operation where everything must be
> > stable, disable OOM killing there and check whether OOM killing
> > happened inbetween and if so back out. 
> 
> This is freeze_processes AFAIU. I might be wrong of course but this is
> the time since when nobody should be waking processes up because they
> could access half suspended devices.

No, you're doing it before freezing starts.  The system is in no way
in a quiescent state at that point.

> > It seems rather obvious to me that OOM killing has to have precedence
> > over PM freezing.
> > 
> > Sure, once the system reaches a point where the whole system must be
> > in a stable state for snapshotting or whatever, disabling OOM killing
> > is fine but at that point the system is in a very limited execution
> > mode and sure won't be processing page faults from userland for
> > example and we can actually disable OOM killing knowing that anything
> > afterwards is ready to handle memory allocation failures.
> 
> I am really confused now. This is basically what the final patch does
> actually.  Here is the what I have currently just to make the further
> discussion easier.

Please see above.

-- 
tejun

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