Re: [PATCH v4 07/25] memcg: Reclaim when more than one page needed.

[Glauber Costa]

 On 06/25/2012 05:13 PM, Glauber Costa wrote:
> > > > > +
> > > > >       ret = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
> > > > >       if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
> > > > >           return CHARGE_RETRY;
> > > > > @@ -2234,8 +2235,10 @@ 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 <= (1 << PAGE_ALLOC_COSTLY_ORDER) && ret) {
> > > > > +        cond_resched();
> > > > >           return CHARGE_RETRY;
> > > > > +    }
> > > >
> > > > What prevents us from looping for unbounded amount of time here?
> > > > Maybe you need to consider the number of reclaimed pages here.
> > >
> > > Why would we even loop here? It will just return CHARGE_RETRY, it is
> > > up to the caller to decide whether or not it will retry.
> >
> > Yes, but the test was original to prevent oom when we managed to reclaim
> > something. And something might be enough for a single page but now you
> > have high order allocations so we can retry without any success.
>
> So,
>
> Most of the kmem allocations are likely to be quite small as well. For
> the slab, we're dealing with the order of 2-3 pages, and for other
> allocations that may happen, like stack, they will be in the order of 2
> pages as well.
>
> So one thing I could do here, is define a threshold, say, 3, and only
> retry for that very low threshold, instead of following COSTLY_ORDER.
> I don't expect two or three pages to be much less likely to be freed
> than a single page.
>
> I am fine with ripping of the cond_resched as well.
>
> Let me know if you would be okay with that.

For the record, here's the patch I would propose.

At this point, I think it would be nice to Suleiman to say if he is 
still okay with the changes.