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

[Michal Hocko]

On Wed 20-06-12 23:43:52, Glauber Costa wrote:
> On 06/20/2012 05:47 PM, Michal Hocko wrote:
> >On Mon 18-06-12 14:28:00, Glauber Costa wrote:
> >>From: Suleiman Souhlal <ssouhlal at FreeBSD.org>
> >>
> >>mem_cgroup_do_charge() was written before slab accounting, and expects
> >>three cases: being called for 1 page, being called for a stock of 32 pages,
> >>or being called for a hugepage.  If we call for 2 or 3 pages (and several
> >>slabs used in process creation are such, at least with the debug options I
> >>had), it assumed it's being called for stock and just retried without reclaiming.
> >>
> >>Fix that by passing down a minsize argument in addition to the csize.
> >>
> >>And what to do about that (csize == PAGE_SIZE && ret) retry?  If it's
> >>needed at all (and presumably is since it's there, perhaps to handle
> >>races), then it should be extended to more than PAGE_SIZE, yet how far?
> >>And should there be a retry count limit, of what?  For now retry up to
> >>COSTLY_ORDER (as page_alloc.c does), stay safe with a cond_resched(),
> >>and make sure not to do it if __GFP_NORETRY.
> >>
> >>[v4: fixed nr pages calculation pointed out by Christoph Lameter ]
> >>
> >>Signed-off-by: Suleiman Souhlal <suleiman at google.com>
> >>Signed-off-by: Glauber Costa <glommer at parallels.com>
> >>Reviewed-by: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> >
> >I think this is not ready to be merged yet.
> Fair Enough
> 
> >Two comments below.
> >
> >[...]
> >>@@ -2210,18 +2211,18 @@ 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;
> >
> >This is kmem specific and should be preparated out in case this should
> >be merged before the rest.
> ok.
> 
> >Btw. I assume that oom==false when called from kmem...
> 
> What prevents the oom killer to be called for a reclaimable kmem
> allocation that can be satisfied ?

Well, I am not familiar with the rest of the patch series yet (sorry
about that) but playing with oom can be really nasty if oom score
doesn't consider also kmem allocations. You can end up killing
unexpected processes just because of kmem hungry (and nasty) process.
Dunno, have to thing about that.

> >>+
> >>  	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.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic