Re: [PATCH] memcg, oom: move out_of_memory back to the charge path

[Michal Hocko]

On Tue 03-07-18 00:08:05, Greg Thelen wrote:
> Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> 
> > On Fri 29-06-18 11:59:04, Greg Thelen wrote:
> >> Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >> 
> >> > On Thu 28-06-18 16:19:07, Greg Thelen wrote:
> >> >> Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >> > [...]
> >> >> > +	if (mem_cgroup_out_of_memory(memcg, mask, order))
> >> >> > +		return OOM_SUCCESS;
> >> >> > +
> >> >> > +	WARN(1,"Memory cgroup charge failed because of no reclaimable memory! "
> >> >> > +		"This looks like a misconfiguration or a kernel bug.");
> >> >> 
> >> >> I'm not sure here if the warning should here or so strongly worded.  It
> >> >> seems like the current task could be oom reaped with MMF_OOM_SKIP and
> >> >> thus mem_cgroup_out_of_memory() will return false.  So there's nothing
> >> >> alarming in that case.
> >> >
> >> > If the task is reaped then its charges should be released as well and
> >> > that means that we should get below the limit. Sure there is some room
> >> > for races but this should be still unlikely. Maybe I am just
> >> > underestimating though.
> >> >
> >> > What would you suggest instead?
> >> 
> >> I suggest checking MMF_OOM_SKIP or deleting the warning.
> >
> > So what do you do when you have MMF_OOM_SKIP task? Do not warn? Checking
> > for all the tasks would be quite expensive and remembering that from the
> > task selection not nice either. Why do you think it would help much?
> 
> I assume we could just check current's MMF_OOM_SKIP - no need to check
> all tasks.

I still do not follow. If you are after a single task memcg then we
should be ok. try_charge has a runaway for oom victims
	if (unlikely(tsk_is_oom_victim(current) ||
		     fatal_signal_pending(current) ||
		     current->flags & PF_EXITING))
		goto force;

regardless of MMF_OOM_SKIP. So if there is a single process in the
memcg, we kill it and the oom reaper kicks in and sets MMF_OOM_SKIP then
we should bail out there. Or do I miss your intention?

> My only (minor) objection is that the warning text suggests
> misconfiguration or kernel bug, when there may be neither.
> 
> > I feel strongly that we have to warn when bypassing the charge limit
> > during the corner case because it can lead to unexpected behavior and
> > users should be aware of this fact. I am open to the wording or some
> > optimizations. I would prefer the latter on top with a clear description
> > how it helped in a particular case though. I would rather not over
> > optimize now without any story to back it.
> 
> I'm fine with the warning.  I know enough to look at dmesg logs to take
> an educates that the race occurred.  We can refine it later if/when the
> reports start rolling in.  No change needed.

OK. Thanks!

-- 
Michal Hocko
SUSE Labs