From: Michal Hocko <mhocko@kernel.org>
To: Roman Gushchin <guro@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Christoph Lameter <cl@linux.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Shakeel Butt <shakeelb@google.com>,
linux-mm@kvack.org, Vlastimil Babka <vbabka@suse.cz>,
kernel-team@fb.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 05/19] mm: memcontrol: decouple reference counting from page accounting
Date: Mon, 3 Aug 2020 11:00:33 +0200 [thread overview]
Message-ID: <20200803090033.GE5174@dhcp22.suse.cz> (raw)
In-Reply-To: <20200623174037.3951353-6-guro@fb.com>
I am sorry for coming late here.
On Tue 23-06-20 10:40:23, Roman Gushchin wrote:
> From: Johannes Weiner <hannes@cmpxchg.org>
>
> The reference counting of a memcg is currently coupled directly to how
> many 4k pages are charged to it. This doesn't work well with Roman's new
> slab controller, which maintains pools of objects and doesn't want to keep
> an extra balance sheet for the pages backing those objects.
>
> This unusual refcounting design (reference counts usually track pointers
> to an object) is only for historical reasons: memcg used to not take any
> css references and simply stalled offlining until all charges had been
> reparented and the page counters had dropped to zero. When we got rid of
> the reparenting requirement, the simple mechanical translation was to take
> a reference for every charge.
>
> More historical context can be found in commit e8ea14cc6ead ("mm:
> memcontrol: take a css reference for each charged page"), commit
> 64f219938941 ("mm: memcontrol: remove obsolete kmemcg pinning tricks") and
> commit b2052564e66d ("mm: memcontrol: continue cache reclaim from offlined
> groups").
>
> The new slab controller exposes the limitations in this scheme, so let's
> switch it to a more idiomatic reference counting model based on actual
> kernel pointers to the memcg:
>
> - The per-cpu stock holds a reference to the memcg its caching
>
> - User pages hold a reference for their page->mem_cgroup. Transparent
> huge pages will no longer acquire tail references in advance, we'll
> get them if needed during the split.
>
> - Kernel pages hold a reference for their page->mem_cgroup
>
> - Pages allocated in the root cgroup will acquire and release css
> references for simplicity. css_get() and css_put() optimize that.
>
> - The current memcg_charge_slab() already hacked around the per-charge
> references; this change gets rid of that as well.
just for completeness
- tcp accounting will handle reference in mem_cgroup_sk_{alloc,free}
As all those paths are handling the reference count differently it is
probably good to remind that in a comment:
/* Caller is responsible to hold reference for the existence of the charged object *
for try_charge function.
We will need to be more careful (e.g. http://lkml.kernel.org/r/alpine.LSU.2.11.2007302011450.2347@eggly.anvils)
but considering that the old model doesn't fit with the new slab
accounting as mentioned above this is not really something terrible to
live with.
[...]
> @@ -5456,7 +5460,10 @@ static int mem_cgroup_move_account(struct page *page,
> */
> smp_mb();
>
> - page->mem_cgroup = to; /* caller should have done css_get */
> + css_get(&to->css);
> + css_put(&from->css);
> +
> + page->mem_cgroup = to;
>
> __unlock_page_memcg(from);
What prevents from memcg to be released here?
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-08-03 9:00 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-23 17:40 [PATCH v7 00/19] The new cgroup slab memory controller Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 01/19] mm: memcg: factor out memcg- and lruvec-level changes out of __mod_lruvec_state() Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 02/19] mm: memcg: prepare for byte-sized vmstat items Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 03/19] mm: memcg: convert vmstat slab counters to bytes Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 04/19] mm: slub: implement SLUB version of obj_to_index() Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 05/19] mm: memcontrol: decouple reference counting from page accounting Roman Gushchin
2020-08-03 9:00 ` Michal Hocko [this message]
2020-08-03 15:03 ` Johannes Weiner
2020-08-03 15:08 ` Michal Hocko
2020-06-23 17:40 ` [PATCH v7 06/19] mm: memcg/slab: obj_cgroup API Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 07/19] mm: memcg/slab: allocate obj_cgroups for non-root slab pages Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 08/19] mm: memcg/slab: save obj_cgroup for non-root slab objects Roman Gushchin
2020-07-16 16:55 ` Naresh Kamboju
2020-07-16 20:07 ` Roman Gushchin
2020-07-17 5:34 ` Naresh Kamboju
2020-06-23 17:40 ` [PATCH v7 10/19] mm: memcg/slab: deprecate memory.kmem.slabinfo Roman Gushchin
2020-06-24 1:43 ` Shakeel Butt
2020-06-24 1:53 ` Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 11/19] mm: memcg/slab: move memcg_kmem_bypass() to memcontrol.h Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 12/19] mm: memcg/slab: use a single set of kmem_caches for all accounted allocations Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 13/19] mm: memcg/slab: simplify memcg cache creation Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 14/19] mm: memcg/slab: remove memcg_kmem_get_cache() Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 15/19] mm: memcg/slab: deprecate slab_root_caches Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 16/19] mm: memcg/slab: remove redundant check in memcg_accumulate_slabinfo() Roman Gushchin
2020-06-23 17:40 ` [PATCH v7 17/19] mm: memcg/slab: use a single set of kmem_caches for all allocations Roman Gushchin
2020-07-18 17:24 ` Guenter Roeck
2020-07-18 23:03 ` Roman Gushchin
-- strict thread matches above, loose matches on Subject: below --
2020-06-23 1:58 [PATCH v7 00/19] The new cgroup slab memory controller Roman Gushchin
2020-06-23 1:58 ` [PATCH v7 05/19] mm: memcontrol: decouple reference counting from page accounting Roman Gushchin
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