Re: [v2 PATCH 2/9] mm: memcontrol: use shrinker_rwsem to protect shrinker_maps allocation

Kirill Tkhai
2020-12-16T13:17:19Z

16.12.2020, 00:59, "Dave Chinner" <david@fromorbit.com>:
> On Tue, Dec 15, 2020 at 02:53:48PM +0100, Johannes Weiner wrote:
>>  On Tue, Dec 15, 2020 at 01:09:57PM +1100, Dave Chinner wrote:
>>  > On Mon, Dec 14, 2020 at 02:37:15PM -0800, Yang Shi wrote:
>>  > > Since memcg_shrinker_map_size just can be changd under holding shrinker_rwsem
>>  > > exclusively, the read side can be protected by holding read lock, so it sounds
>>  > > superfluous to have a dedicated mutex.
>>  >
>>  > I'm not sure this is a good idea. This couples the shrinker
>>  > infrastructure to internal details of how cgroups are initialised
>>  > and managed. Sure, certain operations might be done in certain
>>  > shrinker lock contexts, but that doesn't mean we should share global
>>  > locks across otherwise independent subsystems....
>>
>>  They're not independent subsystems. Most of the memory controller is
>>  an extension of core VM operations that is fairly difficult to
>>  understand outside the context of those operations. Then there are a
>>  limited number of entry points from the cgroup interface. We used to
>>  have our own locks for core VM structures (private page lock e.g.) to
>>  coordinate VM and cgroup, and that was mostly unintelligble.
>
> Yes, but OTOH you can CONFIG_MEMCG=n and the shrinker infrastructure
> and shrinkers all still functions correctly. Ergo, the shrinker
> infrastructure is independent of memcgs. Yes, it may have functions
> to iterate and manipulate memcgs, but it is not dependent on memcgs
> existing for correct behaviour and functionality.
>
> Yet.
>
>>  We have since established that those two components coordinate with
>>  native VM locking and lifetime management. If you need to lock the
>>  page, you lock the page - instead of having all VM paths that already
>>  hold the page lock acquire a nested lock to exclude one cgroup path.
>>
>>  In this case, we have auxiliary shrinker data, subject to shrinker
>>  lifetime and exclusion rules. It's much easier to understand that
>>  cgroup creation needs a stable shrinker list (shrinker_rwsem) to
>>  manage this data, than having an aliased lock that is private to the
>>  memcg callbacks and obscures this real interdependency.
>
> Ok, so the way to do this is to move all the stuff that needs to be
> done under a "subsystem global" lock to the one file, not turn a
> static lock into a globally visible lock and spray it around random
> source files. There's already way too many static globals to manage
> separate shrinker and memcg state..
>
> I certainly agree that shrinkers and memcg need to be more closely
> integrated. I've only been saying that for ... well, since memcgs
> essentially duplicated the top level shrinker path so the shrinker
> map could be introduced to avoid calling shrinkers that have no work
> to do for memcgs. The shrinker map should be generic functionality
> for all shrinker invocations because even a non-memcg machine can
> have thousands of registered shrinkers that are mostly idle all the
> time.
>
> IOWs, I think the shrinker map management is not really memcg
> specific - it's just allocation and assignment of a structure, and
> the only memcg bit is the map is being stored in a memcg structure.
> Therefore, if we are looking towards tighter integration then we
> should acutally move the map management to the shrinker code, not
> split the shrinker infrastructure management across different files.
> There's already a heap of code in vmscan.c under #ifdef
> CONFIG_MEMCG, like the prealloc_shrinker() code path:
>
> prealloc_shrinker() vmscan.c
>   if (MEMCG_AWARE) vmscan.c
>     prealloc_memcg_shrinker vmscan.c
> #ifdef CONFIG_MEMCG vmscan.c
>       down_write(shrinker_rwsem) vmscan.c
>       if (id > shrinker_id_max) vmscan.c
>         memcg_expand_shrinker_maps memcontrol.c
>           for_each_memcg memcontrol.c
>             reallocate shrinker map memcontrol.c
>             replace shrinker map memcontrol.c
>         shrinker_id_max = id vmscan.c
>       down_write(shrinker_rwsem) vmscan.c
> #endif
>
> And, really, there's very little code in memcg_expand_shrinker_maps()
> here - the only memcg part is the memcg iteration loop, and we
> already have them in vmscan.c (e.g. shrink_node_memcgs(),
> age_active_anon(), drop_slab_node()) so there's precedence for
> moving this memcg iteration for shrinker map management all into
> vmscan.c.
>
> Doing so would formalise the shrinker maps as first class shrinker
> infrastructure rather than being tacked on to the side of the memcg
> infrastructure. At this point it makes total sense to serialise map
> manipulations under the shrinker_rwsem.
>
> IOWs, I'm not disagreeing with the direction this patch takes us in,
> I'm disagreeing with the implementation as published in the patch
> because it doesn't move us closer to a clean, concise single
> shrinker infrastructure implementation.
>
> That is, for the medium term, I think we should be getting rid of
> the "legacy" non-memcg shrinker path and everything runs under
> memcgs. With this patchset moving all the deferred counts to be
> memcg aware, the only reason for keeping the non-memcg path around
> goes away. If sc->memcg is null, then after this patch set we can
> simply use the root memcg and just use it's per-node accounting
> rather than having a separate construct for non-memcg aware per-node
> accounting.

Killing "sc->memcg == NULL" cases looks like a great idea. This is equal
to making possible "memory_cgrp_subsys.early_init = 1" with all requirepments
for that, which is a topic for big separate patchset.

> Hence if SHRINKER_MEMCG_AWARE is set, it simply means we should run
> the shrinker if sc->memcg is set. There is no difference in setup
> of shrinkers, the duplicate non-memcg/memcg paths go away, and a
> heap of code drops out of the shrinker infrastructure. It becomes
> much simpler overall.
>
> It also means we have a path for further integrating memcg aware
> shrinkers into the shrinker infrastructure because we can always
> rely on the shrinker infrastructure being memcg aware. And with that
> in mind, I think we should probably also be moving the shrinker code
> out of vmscan.c into it's own file as it's really completely
> separate infrastructure from the vast majority of page reclaim
> infrastructure in vmscan.c...
>
> That's the view I'm looking at this patchset from. Not just as a
> standalone bug fix, but also from the perspective of what the
> architectural change implies and the directions for tighter
> integration it opens up for us.
>
> Cheers,
>
> Dave.
>
> --
> Dave Chinner
> david@fromorbit.com
>
> .