[Devel] Re: [PATCH v4 06/14] memcg: kmem controller infrastructure

[Michal Hocko]

On Fri 12-10-12 11:45:46, Glauber Costa wrote:
> On 10/11/2012 04:42 PM, Michal Hocko wrote:
> > On Mon 08-10-12 14:06:12, Glauber Costa wrote:
[...]
> >> +	/*
> >> +	 * Conditions under which we can wait for the oom_killer.
> >> +	 * __GFP_NORETRY should be masked by __mem_cgroup_try_charge,
> >> +	 * but there is no harm in being explicit here
> >> +	 */
> >> +	may_oom = (gfp & __GFP_WAIT) && !(gfp & __GFP_NORETRY);
> > 
> > Well we _have to_ check __GFP_NORETRY here because if we don't then we
> > can end up in OOM. mem_cgroup_do_charge returns CHARGE_NOMEM for
> > __GFP_NORETRY (without doing any reclaim) and of oom==true we decrement
> > oom retries counter and eventually hit OOM killer. So the comment is
> > misleading.
> 
> I will update. What i understood from your last message is that we don't
> really need to, because try_charge will do it.

IIRC I just said it couldn't happen before because migration doesn't go
through charge and thp disable oom by default.

> >> +
> >> +	_memcg = memcg;
> >> +	ret = __mem_cgroup_try_charge(NULL, gfp, size >> PAGE_SHIFT,
> >> +				      &_memcg, may_oom);
> >> +
> >> +	if (!ret) {
> >> +		ret = res_counter_charge(&memcg->kmem, size, &fail_res);
> > 
> > Now that I'm thinking about the charging ordering we should charge the
> > kmem first because we would like to hit kmem limit before we hit u+k
> > limit, don't we. 
> > Say that you have kmem limit 10M and the total limit 50M. Current `u'
> > would be 40M and this charge would cause kmem to hit the `k' limit. I
> > think we should fail to charge kmem before we go to u+k and potentially
> > reclaim/oom.
> > Or has this been alredy discussed and I just do not remember?
> > 
> This has never been discussed as far as I remember. We charged u first
> since day0, and you are so far the first one to raise it...
> 
> One of the things in favor of charging 'u' first is that
> mem_cgroup_try_charge is already equipped to make a lot of decisions,
> like when to allow reclaim, when to bypass charges, and it would be good
> if we can reuse all that.

Hmm, I think that we should prevent from those decisions if kmem charge
would fail anyway (especially now when we do not have targeted slab
reclaim).

> You oom-based argument makes some sense, if all other scenarios are
> unchanged by this, I can change it. I will give this some more
> consideration.
> 
[...]
> > 	/*
> > 	 * Keep reference on memcg while the page is charged to prevent
> > 	 * group from vanishing because allocation can outlive their
> > 	 * tasks. The reference is dropped in __memcg_kmem_uncharge_page
> > 	 */
> > 
> > please
> 
> I can do that, but keep in mind this piece of code is going away soon =)

Yes I have noticed that and replied to myself that it is not necessary.

-- 
Michal Hocko
SUSE Labs