Re: [RFC PATCH 2/2] memcg: do not report racy no-eligible OOM tasks

[Michal Hocko]

On Tue 23-10-18 21:33:43, Tetsuo Handa wrote:
> On 2018/10/23 21:10, Michal Hocko wrote:
> > On Tue 23-10-18 13:42:46, Michal Hocko wrote:
> >> On Tue 23-10-18 10:01:08, Tetsuo Handa wrote:
> >>> Michal Hocko wrote:
> >>>> On Mon 22-10-18 20:45:17, Tetsuo Handa wrote:
> >>>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >>>>>> index e79cb59552d9..a9dfed29967b 100644
> >>>>>> --- a/mm/memcontrol.c
> >>>>>> +++ b/mm/memcontrol.c
> >>>>>> @@ -1380,10 +1380,22 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >>>>>>  		.gfp_mask = gfp_mask,
> >>>>>>  		.order = order,
> >>>>>>  	};
> >>>>>> -	bool ret;
> >>>>>> +	bool ret = true;
> >>>>>>  
> >>>>>>  	mutex_lock(&oom_lock);
> >>>>>> +
> >>>>>> +	/*
> >>>>>> +	 * multi-threaded tasks might race with oom_reaper and gain
> >>>>>> +	 * MMF_OOM_SKIP before reaching out_of_memory which can lead
> >>>>>> +	 * to out_of_memory failure if the task is the last one in
> >>>>>> +	 * memcg which would be a false possitive failure reported
> >>>>>> +	 */
> >>>>>> +	if (tsk_is_oom_victim(current))
> >>>>>> +		goto unlock;
> >>>>>> +
> >>>>>
> >>>>> This is not wrong but is strange. We can use mutex_lock_killable(&oom_lock)
> >>>>> so that any killed threads no longer wait for oom_lock.
> >>>>
> >>>> tsk_is_oom_victim is stronger because it doesn't depend on
> >>>> fatal_signal_pending which might be cleared throughout the exit process.
> >>>>
> >>>
> >>> I still want to propose this. No need to be memcg OOM specific.
> >>
> >> Well, I maintain what I've said [1] about simplicity and specific fix
> >> for a specific issue. Especially in the tricky code like this where all
> >> the consequences are far more subtle than they seem to be.
> >>
> >> This is obviously a matter of taste but I don't see much point discussing
> >> this back and forth for ever. Unless there is a general agreement that
> >> the above is less appropriate then I am willing to consider a different
> >> change but I simply do not have energy to nit pick for ever.
> >>
> >> [1] http://lkml.kernel.org/r/20181022134315.GF18839@xxxxxxxxxxxxxx
> > 
> > In other words. Having a memcg specific fix means, well, a memcg
> > maintenance burden. Like any other memcg specific oom decisions we
> > already have. So are you OK with that Johannes or you would like to see
> > a more generic fix which might turn out to be more complex?
> > 
> 
> I don't know what "that Johannes" refers to.

let me rephrase

Johannes, are you OK with that (memcg specific fix) or you would like to
see a more generic fix which might turn out to be more complex.
-- 
Michal Hocko
SUSE Labs