Date:   Mon, 27 Jun 2022 13:43:38 +0300
From:   Mika Penttilä <mpenttil@...hat.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>,
        Muchun Song <songmuchun@...edance.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>, longman@...hat.com,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Cgroups <cgroups@...r.kernel.org>, duanxiongchun@...edance.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v6 00/11] Use obj_cgroup APIs to charge the LRU pages



On 27.6.2022 11.05, Yosry Ahmed wrote:
> On Mon, Jun 27, 2022 at 12:11 AM Muchun Song <songmuchun@...edance.com> wrote:
>>
>> On Sun, Jun 26, 2022 at 03:32:02AM -0700, Yosry Ahmed wrote:
>>> On Tue, Jun 21, 2022 at 5:57 AM Muchun Song <songmuchun@...edance.com> wrote:
>>>>
>>>> This version is rebased on mm-unstable. Hopefully, Andrew can get this series
>>>> into mm-unstable which will help to determine whether there is a problem or
>>>> degradation. I am also doing some benchmark tests in parallel.
>>>>
>>>> Since the following patchsets applied. All the kernel memory are charged
>>>> with the new APIs of obj_cgroup.
>>>>
>>>>          commit f2fe7b09a52b ("mm: memcg/slab: charge individual slab objects instead of pages")
>>>>          commit b4e0b68fbd9d ("mm: memcontrol: use obj_cgroup APIs to charge kmem pages")
>>>>
>>>> But user memory allocations (LRU pages) pinning memcgs for a long time -
>>>> it exists at a larger scale and is causing recurring problems in the real
>>>> world: page cache doesn't get reclaimed for a long time, or is used by the
>>>> second, third, fourth, ... instance of the same job that was restarted into
>>>> a new cgroup every time. Unreclaimable dying cgroups pile up, waste memory,
>>>> and make page reclaim very inefficient.
>>>>
>>>> We can convert LRU pages and most other raw memcg pins to the objcg direction
>>>> to fix this problem, and then the LRU pages will not pin the memcgs.
>>>>
>>>> This patchset aims to make the LRU pages to drop the reference to memory
>>>> cgroup by using the APIs of obj_cgroup. Finally, we can see that the number
>>>> of the dying cgroups will not increase if we run the following test script.
>>>
>>> This is amazing work!
>>>
>>> Sorry if I came late, I didn't follow the threads of previous versions
>>> so this might be redundant, I just have a couple of questions.
>>>
>>> a) If LRU pages keep getting parented until they reach root_mem_cgroup
>>> (assuming they can), aren't these pages effectively unaccounted at
>>> this point or leaked? Is there protection against this?
>>>
>>
>> In this case, those pages are accounted in root memcg level. Unfortunately,
>> there is no mechanism now to transfer a page's memcg from one to another.
>>
>>> b) Since moving charged pages between memcgs is now becoming easier by
>>> using the APIs of obj_cgroup, I wonder if this opens the door for
>>> future work to transfer charges to memcgs that are actually using
>>> reparented resources. For example, let's say cgroup A reads a few
>>> pages into page cache, and then they are no longer used by cgroup A.
>>> cgroup B, however, is using the same pages that are currently charged
>>> to cgroup A, so it keeps taxing cgroup A for its use. When cgroup A
>>> dies, and these pages are reparented to A's parent, can we possibly
>>> mark these reparented pages (maybe in the page tables somewhere) so
>>> that next time they get accessed we recharge them to B instead
>>> (possibly asynchronously)?
>>> I don't have much experience about page tables but I am pretty sure
>>> they are loaded so maybe there is no room in PTEs for something like
>>> this, but I have always wondered about what we can do for this case
>>> where a cgroup is consistently using memory charged to another cgroup.
>>> Maybe when this memory is reparented is a good point in time to decide
>>> to recharge appropriately. It would also fix the reparenty leak to
>>> root problem (if it even exists).
>>>
>>
>>  From my point of view, this is going to be an improvement to the memcg
>> subsystem in the future.  IIUC, most reparented pages are page cache
>> pages without be mapped to users. So page tables are not a suitable
>> place to record this information. However, we already have this information
>> in struct obj_cgroup and struct mem_cgroup. If a page's obj_cgroup is not
>> equal to the page's obj_cgroup->memcg->objcg, it means this page have
>> been reparented. I am thinking if a place where a page is mapped (probably
>> page fault patch) or page (cache) is written (usually vfs write path)
>> is suitable to transfer page's memcg from one to another. But need more
> 
> Very good point about unmapped pages, I missed this. Page tables will
> do us no good here. Such a change would indeed require careful thought
> because (like you mentioned) there are multiple points in time where
> it might be suitable to consider recharging the page (e.g. when the
> page is mapped). This could be an incremental change though. Right now
> we have no recharging at all, so maybe we can gradually add recharging
> to suitable paths.
> 
>> thinking, e.g. How to decide if a reparented page needs to be transferred?
> 
> Maybe if (page's obj_cgroup->memcg == root_mem_cgroup) OR (memcg of
> current is not a descendant of page's obj_cgroup->memcg) is a good
> place to start?
> 
> My rationale is that if the page is charged to root_mem_cgroup through
> reparenting and a process in a memcg is using it then this is probably
> an accounting leak. If a page is charged to a memcg A through
> reparenting and is used by a memcg B in a different subtree, then
> probably memcg B is getting away with using the page for free while A
> is being taxed. If B is a descendant of A, it is still getting away
> with using the page unaccounted, but at least it makes no difference
> for A.
> 
> One could argue that we might as well recharge a reparented page
> anyway if the process is cheap (or done asynchronously), and the paths
> where we do recharging are not very common.
> 
> All of this might be moot, I am just thinking out loud. In any way
> this would be future work and not part of this work.
> 


I think you have to uncharge at the reparented parent to keep balances 
right (because parent is hierarchically charged thru page_counter). And 
maybe recharge after that if appropriate.




> 
>> If we need more information to make this decision, where to store those
>> information? This is my primary thoughts on this question.
> 
>>
>> Thanks.
>>
>>> Thanks again for this work and please excuse my ignorance if any part
>>> of what I said doesn't make sense :)
>>>
>>>>
>>>> ```bash
>>>> #!/bin/bash
>>>>
>>>> dd if=/dev/zero of=temp bs=4096 count=1
>>>> cat /proc/cgroups | grep memory
>>>>
>>>> for i in {0..2000}
>>>> do
>>>>          mkdir /sys/fs/cgroup/memory/test$i
>>>>          echo $$ > /sys/fs/cgroup/memory/test$i/cgroup.procs
>>>>          cat temp >> log
>>>>          echo $$ > /sys/fs/cgroup/memory/cgroup.procs
>>>>          rmdir /sys/fs/cgroup/memory/test$i
>>>> done
>>>>
>>>> cat /proc/cgroups | grep memory
>>>>
>>>> rm -f temp log
>>>> ```
>>>>
>>>> v5: https://lore.kernel.org/all/20220530074919.46352-1-songmuchun@bytedance.com/
>>>> v4: https://lore.kernel.org/all/20220524060551.80037-1-songmuchun@bytedance.com/
>>>> v3: https://lore.kernel.org/all/20220216115132.52602-1-songmuchun@bytedance.com/
>>>> v2: https://lore.kernel.org/all/20210916134748.67712-1-songmuchun@bytedance.com/
>>>> v1: https://lore.kernel.org/all/20210814052519.86679-1-songmuchun@bytedance.com/
>>>> RFC v4: https://lore.kernel.org/all/20210527093336.14895-1-songmuchun@bytedance.com/
>>>> RFC v3: https://lore.kernel.org/all/20210421070059.69361-1-songmuchun@bytedance.com/
>>>> RFC v2: https://lore.kernel.org/all/20210409122959.82264-1-songmuchun@bytedance.com/
>>>> RFC v1: https://lore.kernel.org/all/20210330101531.82752-1-songmuchun@bytedance.com/
>>>>
>>>> v6:
>>>>   - Collect Acked-by and Reviewed-by from Roman and Michal Koutný. Thanks.
>>>>   - Rebase to mm-unstable.
>>>>
>>>> v5:
>>>>   - Lots of improvements from Johannes, Roman and Waiman.
>>>>   - Fix lockdep warning reported by kernel test robot.
>>>>   - Add two new patches to do code cleanup.
>>>>   - Collect Acked-by and Reviewed-by from Johannes and Roman.
>>>>   - I didn't replace local_irq_disable/enable() to local_lock/unlock_irq() since
>>>>     local_lock/unlock_irq() takes an parameter, it needs more thinking to transform
>>>>     it to local_lock.  It could be an improvement in the future.
>>>>
>>>> v4:
>>>>   - Resend and rebased on v5.18.
>>>>
>>>> v3:
>>>>   - Removed the Acked-by tags from Roman since this version is based on
>>>>     the folio relevant.
>>>>
>>>> v2:
>>>>   - Rename obj_cgroup_release_kmem() to obj_cgroup_release_bytes() and the
>>>>     dependencies of CONFIG_MEMCG_KMEM (suggested by Roman, Thanks).
>>>>   - Rebase to linux 5.15-rc1.
>>>>   - Add a new pacth to cleanup mem_cgroup_kmem_disabled().
>>>>
>>>> v1:
>>>>   - Drop RFC tag.
>>>>   - Rebase to linux next-20210811.
>>>>
>>>> RFC v4:
>>>>   - Collect Acked-by from Roman.
>>>>   - Rebase to linux next-20210525.
>>>>   - Rename obj_cgroup_release_uncharge() to obj_cgroup_release_kmem().
>>>>   - Change the patch 1 title to "prepare objcg API for non-kmem usage".
>>>>   - Convert reparent_ops_head to an array in patch 8.
>>>>
>>>> Thanks for Roman's review and suggestions.
>>>>
>>>> RFC v3:
>>>>   - Drop the code cleanup and simplification patches. Gather those patches
>>>>     into a separate series[1].
>>>>   - Rework patch #1 suggested by Johannes.
>>>>
>>>> RFC v2:
>>>>   - Collect Acked-by tags by Johannes. Thanks.
>>>>   - Rework lruvec_holds_page_lru_lock() suggested by Johannes. Thanks.
>>>>   - Fix move_pages_to_lru().
>>>>
>>>> Muchun Song (11):
>>>>    mm: memcontrol: remove dead code and comments
>>>>    mm: rename unlock_page_lruvec{_irq, _irqrestore} to
>>>>      lruvec_unlock{_irq, _irqrestore}
>>>>    mm: memcontrol: prepare objcg API for non-kmem usage
>>>>    mm: memcontrol: make lruvec lock safe when LRU pages are reparented
>>>>    mm: vmscan: rework move_pages_to_lru()
>>>>    mm: thp: make split queue lock safe when LRU pages are reparented
>>>>    mm: memcontrol: make all the callers of {folio,page}_memcg() safe
>>>>    mm: memcontrol: introduce memcg_reparent_ops
>>>>    mm: memcontrol: use obj_cgroup APIs to charge the LRU pages
>>>>    mm: lru: add VM_WARN_ON_ONCE_FOLIO to lru maintenance function
>>>>    mm: lru: use lruvec lock to serialize memcg changes
>>>>
>>>>   fs/buffer.c                      |   4 +-
>>>>   fs/fs-writeback.c                |  23 +-
>>>>   include/linux/memcontrol.h       | 218 +++++++++------
>>>>   include/linux/mm_inline.h        |   6 +
>>>>   include/trace/events/writeback.h |   5 +
>>>>   mm/compaction.c                  |  39 ++-
>>>>   mm/huge_memory.c                 | 153 ++++++++--
>>>>   mm/memcontrol.c                  | 584 +++++++++++++++++++++++++++------------
>>>>   mm/migrate.c                     |   4 +
>>>>   mm/mlock.c                       |   2 +-
>>>>   mm/page_io.c                     |   5 +-
>>>>   mm/swap.c                        |  49 ++--
>>>>   mm/vmscan.c                      |  66 ++---
>>>>   13 files changed, 776 insertions(+), 382 deletions(-)
>>>>
>>>>
>>>> base-commit: 882be1ed6b1b5073fc88552181b99bd2b9c0031f
>>>> --
>>>> 2.11.0
>>>>
>>>>
>>>
> 

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