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

[Roman Gushchin <guro@fb.com>]

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.

But let me try to write this down, hopefully v2 will be cleaner.

Thank you!