Re: [PATCH v8 2/9] hugetlb_cgroup: add interface for charge/uncharge hugetlb reservations

[Mike Kravetz]

On 11/25/19 12:26 PM, Mina Almasry wrote:
> On Fri, Nov 8, 2019 at 4:46 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>>
>> On 11/8/19 4:40 PM, Mina Almasry wrote:
>>> On Fri, Nov 8, 2019 at 4:01 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>>>>
>>>> On 11/8/19 3:48 PM, Mina Almasry wrote:
>>>>> On Thu, Nov 7, 2019 at 4:57 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>>>>>>
>>>>>> On 10/29/19 6:36 PM, Mina Almasry wrote:
>>>>>>>
>>>>>>> +static void hugetlb_cgroup_move_parent_reservation(int idx,
>>>>>>> +                                                struct hugetlb_cgroup *h_cg)
>>>>>>> +{
>>>>>>> +     struct hugetlb_cgroup *parent = parent_hugetlb_cgroup(h_cg);
>>>>>>> +
>>>>>>> +     /* Move the reservation counters. */
>>>>>>> +     if (!parent_hugetlb_cgroup(h_cg)) {
>>>>>>> +             parent = root_h_cgroup;
>>>>>>> +             /* root has no limit */
>>>>>>> +             page_counter_charge(
>>>>>>> +                     &root_h_cgroup->reserved_hugepage[idx],
>>>>>>> +                     page_counter_read(
>>>>>>> +                             hugetlb_cgroup_get_counter(h_cg, idx, true)));
>>>>>>> +     }
>>>>>>> +
>>>>>>> +     /* Take the pages off the local counter */
>>>>>>> +     page_counter_cancel(
>>>>>>> +             hugetlb_cgroup_get_counter(h_cg, idx, true),
>>>>>>> +             page_counter_read(hugetlb_cgroup_get_counter(h_cg, idx, true)));
>>>>>>> +}
>>>>>>
>>>>>> I know next to nothing about cgroups and am just comparing this to the
>>>>>> existing hugetlb_cgroup_move_parent() routine.  hugetlb_cgroup_move_parent
>>>>>> updates the cgroup pointer in each page being moved.  Do we need to do
>>>>>> something similar for reservations being moved (move pointer in reservation)?
>>>>>>
>>>>>
>>>>> Oh, good catch. Yes I need to be doing that. I should probably
>>>>> consolidate those routines so the code doesn't miss things like this.
>>>>
>>>> This might get a bit ugly/complicated?  Seems like you will need to examine
>>>> all hugetlbfs inodes and vma's mapping those inodes.
>>>>
>>>
>>> Hmm yes on closer look it does seem like this is not straightforward.
>>> I'll write a test that does this reparenting so I can start running
>>> into the issue and poke for solutions. Off the top of my head, I think
>>> maybe we can just not reparent the hugetlb reservations - the
>>> hugetlb_cgroup stays alive until all its memory is uncharged. That
>>> shouldn't be too bad. Today, I think memcg doesn't reparent memory
>>> when it gets offlined.
>>>
>>> I'll poke at this a bit and come back with suggestions, you may want
>>> to hold off reviewing the rest of the patches until then.
>>
>>
>> Ok, if we start considering what the correct cgroup reparenting semantics
>> should be it would be good to get input from others with more cgroup
>> experience.
>>
> 
> So I looked into this and prototyped a couple of solutions:
> 
> 1. We could repartent the hugetlb reservation using the same approach
> that today we repartent hugetlb faults. Basically today faulted
> hugetlb pages live on hstate->hugepage_activelist. When a hugetlb
> cgroup gets offlined, this list is transversed and any pages on it
> that point to the cgroup being offlined and reparented. hugetlb_lock
> is used to make sure cgroup offlining doesn't race with a page being
> freed. I can add another list, but one that has pointers to the
> reservations made. When the cgroup is being offlined, it transverses
> this list, and reparents any reservations (which will need to acquire
> the proper resv_map->lock to do the parenting). hugetlb_lock needs
> also to be acquired here to make sure that resv_map release doesn't
> race with another thread reparenting the memory in that resv map.
> 
> Pros: Maintains current parenting behavior, and makes sure that
> reparenting of reservations works exactly the same way as reparenting
> of hugetlb faults.
> Cons: Code is a bit complex. There may be subtle object lifetime bugs,
> since I'm not 100% sure acquiring hugetlb_lock removes all races.
> 
> 2. We could just not reparent hugetlb reservations. I.e. on hugetlb
> cgroup offlining, the hugetlb faults get reparented (which maintains
> current user facing behavior), but hugetlb reservation charges remain
> charged to the hugetlb cgroup. The cgroup lives as a zombie until all
> the reservations are uncharged.
> 
> Pros: Much easier implementation. Converges behavior of memcg and
> hugetlb cgroup, since memcg also doesn't reparent memory charged to
> it.
> Cons: Behavior change as hugetlb cgroups will become zombies if there
> are reservations charged to them. I've discussed offlist with Shakeel,
> and AFAICT there are absolutely no user facing behavior change to
> zombie cgroups. Only if the user is specifically detecting for
> zombies.
> 
> I'm torn between these 2 options right now, but leaning towards #2. I
> think I will propose #2 in a patch for review, and if anyone is broken
> by that (again, my understanding is that is very unlikely), then I
> propose a patch that reverts the changes in #2 and implements the
> changes in #1.

I of course like option #2 because it introduces fewer (if any) additional
changes to the hugetlb reservation code for non-cgroup users. :)

> Any feedback from Shakeel or other people with cgroup expertise
> (especially for hugetlb cgroup or memcg)  is very useful here.

Yes, that would be very helpful.

-- 
Mike Kravetz