From: Mike Kravetz <mike.kravetz@oracle.com> To: Mina Almasry <almasrymina@google.com>, Shakeel Butt <shakeelb@google.com>, David Rientjes <rientjes@google.com> Cc: shuah <shuah@kernel.org>, open list <linux-kernel@vger.kernel.org>, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, cgroups@vger.kernel.org, Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com> Subject: Re: [PATCH v8 2/9] hugetlb_cgroup: add interface for charge/uncharge hugetlb reservations Date: Mon, 25 Nov 2019 16:05:06 -0800 [thread overview] Message-ID: <3b65b0ac-54ed-6e0d-706c-358d85057403@oracle.com> (raw) In-Reply-To: <CAHS8izMFAYTgxym-Hzb_JmkTK1N_S9tGN71uS6MFV+R7swYu5A@mail.gmail.com> On 11/25/19 12:26 PM, Mina Almasry wrote: > On Fri, Nov 8, 2019 at 4:46 PM Mike Kravetz <mike.kravetz@oracle.com> wrote: >> >> On 11/8/19 4:40 PM, Mina Almasry wrote: >>> On Fri, Nov 8, 2019 at 4:01 PM Mike Kravetz <mike.kravetz@oracle.com> wrote: >>>> >>>> On 11/8/19 3:48 PM, Mina Almasry wrote: >>>>> On Thu, Nov 7, 2019 at 4:57 PM Mike Kravetz <mike.kravetz@oracle.com> 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
next prev parent reply other threads:[~2019-11-26 0:05 UTC|newest] Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-10-30 1:36 [PATCH v8 1/9] hugetlb_cgroup: Add hugetlb_cgroup reservation counter Mina Almasry 2019-10-30 1:36 ` [PATCH v8 2/9] hugetlb_cgroup: add interface for charge/uncharge hugetlb reservations Mina Almasry 2019-11-08 0:57 ` Mike Kravetz 2019-11-08 23:48 ` Mina Almasry 2019-11-09 0:01 ` Mike Kravetz 2019-11-09 0:40 ` Mina Almasry 2019-11-09 0:46 ` Mike Kravetz 2019-11-25 20:26 ` Mina Almasry 2019-11-26 0:05 ` Mike Kravetz [this message] 2019-10-30 1:36 ` [PATCH v8 3/9] hugetlb_cgroup: add cgroup-v2 support Mina Almasry 2019-10-30 1:36 ` [PATCH v8 4/9] hugetlb_cgroup: add reservation accounting for private mappings Mina Almasry 2019-10-30 1:36 ` [PATCH v8 5/9] hugetlb: disable region_add file_region coalescing Mina Almasry 2019-11-01 23:23 ` Mike Kravetz 2019-11-04 21:04 ` Mina Almasry 2019-11-04 21:15 ` Mike Kravetz 2019-11-04 21:19 ` Mina Almasry 2019-11-17 11:03 ` Wenkuan Wang 2019-11-18 19:41 ` Mina Almasry 2019-10-30 1:36 ` [PATCH v8 6/9] hugetlb_cgroup: add accounting for shared mappings Mina Almasry 2019-10-30 1:36 ` [PATCH v8 7/9] hugetlb_cgroup: support noreserve mappings Mina Almasry 2019-10-30 1:37 ` [PATCH v8 8/9] hugetlb_cgroup: Add hugetlb_cgroup reservation tests Mina Almasry 2019-10-30 1:37 ` [PATCH v8 9/9] hugetlb_cgroup: Add hugetlb_cgroup reservation docs Mina Almasry 2019-11-07 23:42 ` [PATCH v8 1/9] hugetlb_cgroup: Add hugetlb_cgroup reservation counter Mike Kravetz 2019-11-08 23:35 ` Mina Almasry
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