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

[Glauber Costa <glommer@parallels.com>]

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On 10/12/2012 12:57 PM, Michal Hocko wrote:
> On Fri 12-10-12 12:44:57, Glauber Costa wrote:
>> 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);
> 
> OK
> 
>>
>>>>>> +
>>>>>> +	_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.
> 
> Yes and it should work as advertised (aka hit the k limit first).
> 
Just so we don't ping-pong in another submission:

I changed memcontrol.h's memcg_kmem_newpage_charge to include:

        /* If the test is dying, just let it go. */
        if (unlikely(test_thread_flag(TIF_MEMDIE)
                     || fatal_signal_pending(current)))
                return true;


I'm also attaching the proposed code in memcontrol.c


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diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c32aaaf..72cf189 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
+static int memcg_charge_kmem(struct mem_cgroup *memcg, gfp_t gfp, u64 size)
+{
+	struct res_counter *fail_res;
+	struct mem_cgroup *_memcg;
+	int ret = 0;
+	bool may_oom;
+
+	ret = res_counter_charge(&memcg->kmem, size, &fail_res);
+	if (ret)
+		return ret;
+
+	/*
+	 * 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 == -EINTR)  {
+		/*
+		 * __mem_cgroup_try_charge() chosed to bypass to root due to
+		 * OOM kill or fatal signal.  Since our only options are to
+		 * either fail the allocation or charge it to this cgroup, do
+		 * it as a temporary condition. But we can't fail. From a
+		 * kmem/slab perspective, the cache has already been selected,
+		 * by mem_cgroup_get_kmem_cache(), so it is too late to change
+		 * our minds. This condition will only trigger if the task
+		 * entered memcg_charge_kmem in a sane state, but was
+		 * OOM-killed.  during __mem_cgroup_try_charge. Tasks that are
+		 * already dying when the allocation triggers should have been
+		 * already directed to the root cgroup.
+		 */
+		res_counter_charge_nofail(&memcg->res, size, &fail_res);
+		if (do_swap_account)
+			res_counter_charge_nofail(&memcg->memsw, size,
+						  &fail_res);
+		ret = 0;
+	} else if (ret)
+		res_counter_uncharge(&memcg->kmem, size);
+
+	return ret;
+}