Re: [PATCH 02/11] memcg: Reclaim when more than one page needed.

[Glauber Costa]

> > + * retries
> > + */
> > +#define NR_PAGES_TO_RETRY 2
> > +
>
> Should be 1 << PAGE_ALLOC_COSTLY_ORDER?  Where does this number come from?
> The changelog doesn't specify.

Hocko complained about that, and I changed. Where the number comes from, 
is stated in the comments: it is a number small enough to have high 
changes of had been freed by the previous reclaim, and yet around the 
number of pages of a kernel allocation.

Of course there are allocations for nr_pages > 2. But 2 will already 
service the stack most of the time, and most of the slab caches.

> >   static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > -				unsigned int nr_pages, bool oom_check)
> > +				unsigned int nr_pages, unsigned int min_pages,
> > +				bool oom_check)
> >   {
> >   	unsigned long csize = nr_pages * PAGE_SIZE;
> >   	struct mem_cgroup *mem_over_limit;
> > @@ -2182,18 +2190,18 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >   	} else
> >   		mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
> >   	/*
> > -	 * nr_pages can be either a huge page (HPAGE_PMD_NR), a batch
> > -	 * of regular pages (CHARGE_BATCH), or a single regular page (1).
> > -	 *
> >   	 * Never reclaim on behalf of optional batching, retry with a
> >   	 * single page instead.
> >   	 */
> > -	if (nr_pages == CHARGE_BATCH)
> > +	if (nr_pages > min_pages)
> >   		return CHARGE_RETRY;
> >
> >   	if (!(gfp_mask & __GFP_WAIT))
> >   		return CHARGE_WOULDBLOCK;
> >
> > +	if (gfp_mask & __GFP_NORETRY)
> > +		return CHARGE_NOMEM;
> > +
> >   	ret = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
> >   	if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
> >   		return CHARGE_RETRY;
> > @@ -2206,7 +2214,7 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >   	 * unlikely to succeed so close to the limit, and we fall back
> >   	 * to regular pages anyway in case of failure.
> >   	 */
> > -	if (nr_pages == 1 && ret)
> > +	if (nr_pages <= NR_PAGES_TO_RETRY && ret)
> >   		return CHARGE_RETRY;
> >
> >   	/*
> > @@ -2341,7 +2349,8 @@ again:
> >   			nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
> >   		}
> >
> > -		ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, oom_check);
> > +		ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, nr_pages,
> > +		    oom_check);
> >   		switch (ret) {
> >   		case CHARGE_OK:
> >   			break;


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