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

Glauber Costa
2012-10-12T09:13:25Z

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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;
+}