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

[Glauber Costa <glommer@parallels.com>]

On 10/12/2012 12:39 PM, Michal Hocko wrote:
> 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.
> 

I had it changed to:

        /*
         * Conditions under which we can wait for the oom_killer.
         * We have to be able to wait, but also, if we can't retry,
         * we obviously shouldn't go mess with oom.
         */
        may_oom = (gfp & __GFP_WAIT) && !(gfp & __GFP_NORETRY);

>>>> +
>>>> +	_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).
>

Let's revisit this discussion when we do have targeted reclaim. For now,
I'll agree that charging kmem first would be acceptable.

This will only make a difference when K < U anyway.