Re: [PATCH -mm v15 00/13] kmemcg shrinkers

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Feb 13, 2014 at 09:33:32PM +0400, Vladimir Davydov wrote:
> On 02/13/2014 02:01 AM, Johannes Weiner wrote:
> > On Wed, Feb 12, 2014 at 10:05:43PM +0400, Vladimir Davydov wrote:
> >> On 02/12/2014 12:19 AM, Johannes Weiner wrote:
> >>> On Tue, Feb 11, 2014 at 07:15:26PM +0400, Vladimir Davydov wrote:
> >>>> Hi Michal, Johannes, David,
> >>>>
> >>>> Could you please take a look at this if you have time? Without your
> >>>> review, it'll never get committed.
> >>> There is simply no review bandwidth for new features as long as we are
> >>> fixing fundamental bugs in memcg.
> >>>
> >>>> On 02/05/2014 10:39 PM, Vladimir Davydov wrote:
> >>>>> Hi,
> >>>>>
> >>>>> This is the 15th iteration of Glauber Costa's patch-set implementing slab
> >>>>> shrinking on memcg pressure. The main idea is to make the list_lru structure
> >>>>> used by most FS shrinkers per-memcg. When adding or removing an element from a
> >>>>> list_lru, we use the page information to figure out which memcg it belongs to
> >>>>> and relay it to the appropriate list. This allows scanning kmem objects
> >>>>> accounted to different memcgs independently.
> >>>>>
> >>>>> Please note that this patch-set implements slab shrinking only when we hit the
> >>>>> user memory limit so that kmem allocations will still fail if we are below the
> >>>>> user memory limit, but close to the kmem limit. I am going to fix this in a
> >>>>> separate patch-set, but currently it is only worthwhile setting the kmem limit
> >>>>> to be greater than the user mem limit just to enable per-memcg slab accounting
> >>>>> and reclaim.
> >>>>>
> >>>>> The patch-set is based on top of v3.14-rc1-mmots-2014-02-04-16-48 (there are
> >>>>> some vmscan cleanups that I need committed there) and organized as follows:
> >>>>>  - patches 1-4 introduce some minor changes to memcg needed for this set;
> >>>>>  - patches 5-7 prepare fs for per-memcg list_lru;
> >>>>>  - patch 8 implement kmemcg reclaim core;
> >>>>>  - patch 9 make list_lru per-memcg and patch 10 marks sb shrinker memcg-aware;
> >>>>>  - patch 10 is trivial - it issues shrinkers on memcg destruction;
> >>>>>  - patches 12 and 13 introduce shrinking of dead kmem caches to facilitate
> >>>>>    memcg destruction.
> >>> In the context of the ongoing discussions about charge reparenting I
> >>> was curious how you deal with charges becoming unreclaimable after a
> >>> memcg has been offlined.
> >>>
> >>> Patch #11 drops all charged objects at offlining by just invoking
> >>> shrink_slab() in a loop until "only a few" (10) objects are remaining.
> >>> How long is this going to take?  And why is it okay to destroy these
> >>> caches when somebody else might still be using them?
> >> IMHO, on container destruction we have to drop as many objects accounted
> >> to this container as we can, because otherwise any container will be
> >> able to get access to any number of unaccounted objects by fetching them
> >> and then rebooting.
> > They're accounted to and subject to the limit of the parent.  I don't
> > see how this is different than page cache.
> >
> >>> That still leaves you with the free objects that slab caches retain
> >>> for allocation efficiency, so now you put all dead memcgs in the
> >>> system on a global list, and on a vmpressure event on root_mem_cgroup
> >>> you walk the global list and drain the freelist of all remaining
> >>> caches.
> >>>
> >>> This is a lot of complexity and scalability problems for less than
> >>> desirable behavior.
> >>>
> >>> Please think about how we can properly reparent kmemcg charges during
> >>> memcg teardown.  That would simplify your code immensely and help
> >>> clean up this unholy mess of css pinning.
> >>>
> >>> Slab caches are already collected in the memcg and on destruction
> >>> could be reassigned to the parent.  Kmemcg uncharge from slab freeing
> >>> would have to be updated to use the memcg from the cache, not from the
> >>> individual page, but I don't see why this wouldn't work right now.
> >> I don't think I understand what you mean by reassigning slab caches to
> >> the parent.
> >>
> >> If you mean moving all pages (slabs) from the cache of the memcg being
> >> destroyed to the corresponding root cache (or the parent memcg's cache)
> >> and then destroying the memcg's cache, I don't think this is feasible,
> >> because slub free's fast path is lockless, so AFAIU we can't remove a
> >> partial slab from a cache w/o risking to race with kmem_cache_free.
> >>
> >> If you mean clearing all pointers from the memcg's cache to the memcg
> >> (changing them to the parent or root memcg), then AFAIU this won't solve
> >> the problem with "dangling" caches - we will still have to shrink them
> >> on vmpressure. So although this would allow us to put the reference to
> >> the memcg from kmem caches on memcg's death, it wouldn't simplify the
> >> code at all, in fact, it would even make it more complicated, because we
> >> would have to handle various corner cases like reparenting vs
> >> list_lru_{add,remove}.
> > I think we have different concepts of what's complicated.  There is an
> > existing model of what to do with left-over cache memory when a cgroup
> > is destroyed, which is reparenting.  The rough steps will be the same,
> > the object lifetime will be the same, the css refcounting will be the
> > same, the user-visible behavior will be the same.  Any complexity from
> > charge vs. reparent races will be contained to a few lines of code.
> >
> > Weird refcounting tricks during offline, trashing kmem caches instead
> > of moving them to the parent like other memory, a global list of dead
> > memcgs and sudden freelist thrashing on a vmpressure event, that's what
> > adds complexity and what makes this code unpredictable, fragile, and
> > insanely hard to work with.  It's not acceptable.
> >
> > By reparenting I meant reassigning the memcg cache parameter pointer
> > from the slab cache such that it points to the parent.  This should be
> > an atomic operation.  All css lookups already require RCU (I think slab
> > does not follow this yet because we guarantee that css reference, but
> > it should be changed).  So switch the cache param pointer, insert an
> > RCU graceperiod to wait for all the ongoing charges and uncharges until
> > nobody sees the memcg anymore, then safely reparent all the remaining
> > memcg objects to the parent.  Maybe individually, maybe we can just
> > splice the lists to the parent's list_lru lists.
> 
> But what should we do with objects that do not reside on any list_lru?
> How can we reparent them?

If there are no actual list_lru objects, we only have to make sure
that any allocations are properly uncharged against the parent when
they get freed later.

If the slab freeing path would uncharge against the per-memcg cache's
backpointer (s->memcg_params->memcg) instead of the per-page memcg
association, then we could reparent whole caches with a single pointer
update, without touching each individual slab page.  The kmemcg
interface for slab would have to be reworked to not use
lookup_page_cgroup() but have slab pass s->memcg_params->memcg.

Once that is in place, css_free() can walk memcg->memcg_slab_caches
and move all the items to the parent's memcg_slab_caches, and while
doing that change the memcg pointer of each item, memcg_params->memcg,
to point to the parent.  The cache is now owned by the parent and will
stay alive until the last page is freed.

There won't be any new allocations in these caches because there are
no tasks in the group anymore, so no races from that side, and the
perfect time to shrink the freelists.

There will be racing frees of outstanding allocations, but we can deal
with that.  Frees use page->slab_cache to look up the proper per-memcg
cache, which may or may not have been reparented at this time.  If it
has not, the cache's memcg backpointer (s->memcg_params->memcg) will
point to the dying child (rcu protected) and uncharge the child's
res_counter and the parent's.  If the backpointer IS already pointing
to the parent, it will uncharge the res_counter of the parent without
the child's but we don't care, it's dying anyway.

If there are no list_lru tracked objects, we are done at this point.
The css can be freed, the freelists have been purged, and any pages
still in the cache will get unaccounted properly from the parent.

If there are list_lru objects, they have to be moved to the parent's
list_lru so that they can be reclaimed properly on memory pressure.

Does this make sense?

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