On Wed, Dec 16, 2020 at 08:59:38AM +1100, Dave Chinner wrote: > 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. Okay, but now do it the other way round and explain the memcg bits in a world where shrinkers don't exist ;-) Anyway, we seem to be mostly in agreement below. > > 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. Sure, that works as well. > 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. Agreed. > 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. Yes, that's a great idea. > 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. > > 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. Agreed as well. > 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... Right again. > 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. Makes sense, but I'm not sure it's getting in the way of that: a generalized first-class map would be managed under the shrinker_rwsem, so ditching the private lock is good progress. The widened lock scope (temporarily, and still mm/) is easy to reverse later on. That said, moving the map handling code from memcontrol.c to vmscan.c in preparation, and/or even reworking the shrinker around the concept of a memcg, indeed are great ideas. I'd support patches doing that.
