From: Johannes Weiner <hannes@cmpxchg.org>
To: Roman Gushchin <guro@fb.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
Michal Hocko <mhocko@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Kernel Team <Kernel-team@fb.com>,
Shakeel Butt <shakeelb@google.com>,
Vladimir Davydov <vdavydov.dev@gmail.com>,
Waiman Long <longman@redhat.com>
Subject: Re: [PATCH RFC 01/14] mm: memcg: subpage charging API
Date: Tue, 17 Sep 2019 10:50:04 +0200 [thread overview]
Message-ID: <20190917085004.GA1486@cmpxchg.org> (raw)
In-Reply-To: <20190917022713.GB8073@castle.DHCP.thefacebook.com>
On Tue, Sep 17, 2019 at 02:27:19AM +0000, Roman Gushchin wrote:
> On Mon, Sep 16, 2019 at 02:56:11PM +0200, Johannes Weiner wrote:
> > On Thu, Sep 05, 2019 at 02:45:45PM -0700, Roman Gushchin wrote:
> > > Introduce an API to charge subpage objects to the memory cgroup.
> > > The API will be used by the new slab memory controller. Later it
> > > can also be used to implement percpu memory accounting.
> > > In both cases, a single page can be shared between multiple cgroups
> > > (and in percpu case a single allocation is split over multiple pages),
> > > so it's not possible to use page-based accounting.
> > >
> > > The implementation is based on percpu stocks. Memory cgroups are still
> > > charged in pages, and the residue is stored in perpcu stock, or on the
> > > memcg itself, when it's necessary to flush the stock.
> >
> > Did you just implement a slab allocator for page_counter to track
> > memory consumed by the slab allocator?
>
> :)
>
> >
> > > @@ -2500,8 +2577,9 @@ void mem_cgroup_handle_over_high(void)
> > > }
> > >
> > > static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > > - unsigned int nr_pages)
> > > + unsigned int amount, bool subpage)
> > > {
> > > + unsigned int nr_pages = subpage ? ((amount >> PAGE_SHIFT) + 1) : amount;
> > > unsigned int batch = max(MEMCG_CHARGE_BATCH, nr_pages);
> > > int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
> > > struct mem_cgroup *mem_over_limit;
> > > @@ -2514,7 +2592,9 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > > if (mem_cgroup_is_root(memcg))
> > > return 0;
> > > retry:
> > > - if (consume_stock(memcg, nr_pages))
> > > + if (subpage && consume_subpage_stock(memcg, amount))
> > > + return 0;
> > > + else if (!subpage && consume_stock(memcg, nr_pages))
> > > return 0;
> >
> > The layering here isn't clean. We have an existing per-cpu cache to
> > batch-charge the page counter. Why does the new subpage allocator not
> > sit on *top* of this, instead of wedged in between?
> >
> > I think what it should be is a try_charge_bytes() that simply gets one
> > page from try_charge() and then does its byte tracking, regardless of
> > how try_charge() chooses to implement its own page tracking.
> >
> > That would avoid the awkward @amount + @subpage multiplexing, as well
> > as annotating all existing callsites of try_charge() with a
> > non-descript "false" parameter.
> >
> > You can still reuse the stock data structures, use the lower bits of
> > stock->nr_bytes for a different cgroup etc., but the charge API should
> > really be separate.
>
> Hm, I kinda like the idea, however there is a complication: for the subpage
> accounting the css reference management is done in a different way, so that
> all existing code should avoid changing the css refcounter. So I'd need
> to pass a boolean argument anyway.
Can you elaborate on the refcounting scheme? I don't quite understand
how there would be complications with that.
Generally, references are held for each page that is allocated in the
page_counter. try_charge() allocates a batch of css references,
returns one and keeps the rest in stock.
So couldn't the following work?
When somebody allocates a subpage, the css reference returned by
try_charge() is shared by the allocated subpage object and the
remainder that is kept via stock->subpage_cache and stock->nr_bytes
(or memcg->nr_stocked_bytes when the percpu cache is reset).
When the subpage objects are freed, you'll eventually have a full page
again in stock->nr_bytes, at which point you page_counter_uncharge()
paired with css_put(_many) as per usual.
A remainder left in old->nr_stocked_bytes would continue to hold on to
one css reference. (I don't quite understand who is protecting this
remainder in your current version, actually. A bug?)
Instead of doing your own batched page_counter uncharging in
refill_subpage_stock() -> drain_subpage_stock(), you should be able to
call refill_stock() when stock->nr_bytes adds up to a whole page again.
Again, IMO this would be much cleaner architecture if there was a
try_charge_bytes() byte allocator that would sit on top of a cleanly
abstracted try_charge() page allocator, just like the slab allocator
is sitting on top of the page allocator - instead of breaking through
the abstraction layer of the underlying page allocator.
next prev parent reply other threads:[~2019-09-17 8:50 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-05 21:45 [PATCH RFC 00/14] The new slab memory controller Roman Gushchin
2019-09-05 21:45 ` [PATCH RFC 01/14] mm: memcg: subpage charging API Roman Gushchin
2019-09-16 12:56 ` Johannes Weiner
2019-09-17 2:27 ` Roman Gushchin
2019-09-17 8:50 ` Johannes Weiner [this message]
2019-09-17 18:33 ` Roman Gushchin
2019-09-05 21:45 ` [PATCH RFC 02/14] mm: memcg: introduce mem_cgroup_ptr Roman Gushchin
2019-09-05 22:34 ` Roman Gushchin
2019-09-05 21:45 ` [PATCH RFC 03/14] mm: vmstat: use s32 for vm_node_stat_diff in struct per_cpu_nodestat Roman Gushchin
2019-09-05 21:45 ` [PATCH RFC 04/14] mm: vmstat: convert slab vmstat counter to bytes Roman Gushchin
2019-09-16 12:38 ` Johannes Weiner
2019-09-17 2:08 ` Roman Gushchin
2019-09-05 21:45 ` [PATCH RFC 05/14] mm: memcg/slab: allocate space for memcg ownership data for non-root slabs Roman Gushchin
2019-09-05 21:45 ` [PATCH RFC 06/14] mm: slub: implement SLUB version of obj_to_index() Roman Gushchin
2019-09-05 21:45 ` [PATCH RFC 07/14] mm: memcg/slab: save memcg ownership data for non-root slab objects Roman Gushchin
2019-09-05 21:45 ` [PATCH RFC 08/14] mm: memcg: move memcg_kmem_bypass() to memcontrol.h Roman Gushchin
2019-09-05 21:45 ` [PATCH RFC 09/14] mm: memcg: introduce __mod_lruvec_memcg_state() Roman Gushchin
2019-09-05 22:37 ` [PATCH RFC 02/14] mm: memcg: introduce mem_cgroup_ptr Roman Gushchin
2019-09-17 19:48 ` [PATCH RFC 00/14] The new slab memory controller Waiman Long
2019-09-17 21:24 ` Roman Gushchin
2019-09-19 13:39 ` Suleiman Souhlal
2019-09-19 16:22 ` Roman Gushchin
2019-09-19 21:10 ` Suleiman Souhlal
2019-09-19 21:40 ` Roman Gushchin
2019-10-01 15:12 ` Michal Koutný
2019-10-02 2:09 ` Roman Gushchin
2019-10-02 13:00 ` Suleiman Souhlal
2019-10-03 10:47 ` Michal Koutný
2019-10-03 15:52 ` Roman Gushchin
2019-12-09 9:17 ` [PATCH 00/16] " Bharata B Rao
2019-12-09 11:56 ` Bharata B Rao
2019-12-09 18:04 ` Roman Gushchin
2019-12-10 6:23 ` Bharata B Rao
2019-12-10 18:05 ` Roman Gushchin
2020-01-13 8:47 ` Bharata B Rao
2020-01-13 15:31 ` Roman Gushchin
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