linux-kernel - Re: [PATCH RFC 01/14] mm: memcg: subpage charging API

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Date:   Tue, 17 Sep 2019 10:50:04 +0200
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH RFC 01/14] mm: memcg: subpage charging API

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.