Re: OOM killer changes

[Michal Hocko <mhocko@suse.cz>]

On Fri 19-08-16 09:47:59, Vlastimil Babka wrote:
> On 08/19/2016 09:33 AM, Michal Hocko wrote:
> > On Fri 19-08-16 08:27:34, Vlastimil Babka wrote:
> >> On 08/19/2016 04:42 AM, Ralf-Peter Rohbeck wrote:
> >>> On 18.08.2016 13:12, Vlastimil Babka wrote:
> >>>> On 18.8.2016 22:01, Ralf-Peter Rohbeck wrote:
> >>>>> On 17.08.2016 23:57, Vlastimil Babka wrote:
> >>>>>> Vlastimil
> >>>>> Yes, that change was in my test with linux-next-20160817. Here's the diff:
> >>>>>
> >>>>> diff --git a/mm/compaction.c b/mm/compaction.c
> >>>>> index f94ae67..60a9ca2 100644
> >>>>> --- a/mm/compaction.c
> >>>>> +++ b/mm/compaction.c
> >>>>> @@ -1083,8 +1083,10 @@ static void isolate_freepages(struct
> >>>>> compact_control *cc)
> >>>>>                           continue;
> >>>>>
> >>>>>                   /* Check the block is suitable for migration */
> >>>>> +/*
> >>>>>                   if (!suitable_migration_target(page))
> >>>>>                           continue;
> >>>>> +*/
> >>>> OK, could you please also try if uncommenting the above still works without OOM?
> >>>> Or just plain linux-next-20160817, I guess we don't need the printk's to test
> >>>> this difference.
> >>>>
> >>>> Thanks a lot!
> >>>> Vlastimil
> >>>>
> >>> With the two lines back in I had OOMs again. See the attached logs.
> >>
> >> Thanks for the confirmation.
> >>
> >> We however shouldn't disable the heuristic completely, so here's a compromise
> >> patch hooking into the new compaction priorities. Can you please test on top of
> >> linux-next?
> >>
> >> -----8<-----
> >> >From 0927cc2a4c6a3247111168eace9012c23d06f9db Mon Sep 17 00:00:00 2001
> >> From: Vlastimil Babka <vbabka@suse.cz>
> >> Date: Thu, 18 Aug 2016 16:01:14 +0200
> >> Subject: [PATCH] mm, compaction: make full priority ignore pageblock
> >>  suitability
> >>
> >> Ralf-Peter Rohbeck has reported premature OOMs for order-2 allocations (stack)
> >> due to OOM rework in 4.7. In his scenario (parallel kernel build and dd writing
> >> to two drives) many pageblocks get marked as Unmovable and compaction free
> >> scanner struggles to isolate free pages. Joonsoo Kim pointed out that the free
> >> scanner skips pageblocks that are not movable to prevent filling them and
> >> forcing non-movable allocations to fallback to other pageblocks. Such heuristic
> >> makes sense to help prevent long-term fragmentation, but premature OOMs are
> >> relatively more urgent problem. As a compromise, this patch disables the
> >> heuristic only for the ultimate compaction priority.
> >>
> >> Reported-by: Ralf-Peter Rohbeck <Ralf-Peter.Rohbeck@quantum.com>
> >> Suggested-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> >> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> > 
> > Thanks to both of you! I do agree that we should drop all these
> > heuristics when we struggle and there is an OOM risk. I have just a
> > small nit here. I would prefer
> > s@COMPACT_PRIO_SYNC_FULL@MIN_COMPACT_PRIORITY@ when disabling them
> > because this would be easier to follow and it would be easier for future
> > changes.
> 
> OK, but then we should start with a change to
> mm-compaction-add-the-ultimate-direct-compaction-priority.patch
> (fix at the end of this e-mail) to make things consistent.
> Then I will apply that to the new patch if it's successfully tested.

This can go as a separate clean up patch. No need to alter previous
patches sitting in the mmotm.
 
> > Which brings me to another thing I was suggesting earlier. I
> > believe we should go to this MIN_COMPACT_PRIORITY only for !costly
> > requests because costly orders shouldn't get all those exceptions and
> > risk long term fragmentation issues. We do not have that many costly
> > requests (except for hugetlb) so it doesn't matter all that much right
> > now but long term we want to differentiate those I believe.
> 
> I'll send such change afterwards as well.

Thanks!

> > That being said, let's wait for the feedback on this patch + linux-next.
> > If it works out I will send a stable 4.7 patch which drops compaction
> > feedback from should_compact_retry (turn it to the !COMPACTION version)
> > so that 4.7 users do not suffer from the premature OOM and will ask
> > Andrew to sneak the compaction patches to 4.8 as they fix a real issue
> > and the risk is not really high.
> 
> Agreed.
> 
> > Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Thanks!
> 
> -----8<-----
> >From c4da7022e85e52f5463055cdc474656652e7a504 Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@suse.cz>
> Date: Fri, 19 Aug 2016 09:40:31 +0200
> Subject: [PATCH] mm, compaction: add the ultimate direct compaction
>  priority-fix
> 
> Use the MIN_COMPACT_PRIORITY alias instead of COMPACT_PRIO_SYNC_FULL to
> disable heuristics "because this would be easier to follow and it would be
> easier for future changes", per Michal.
> 
> Suggested-by: Michal Hocko <mhocko@suse.cz>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Fixes: mmotm mm-compaction-add-the-ultimate-direct-compaction-priority.patch

I guess Fixes is a bit misleading. This is not a bug it is a cleanup
patch.

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  mm/compaction.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ae4f40afcca1..3e35fce2cace 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1644,8 +1644,8 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
>  		.alloc_flags = alloc_flags,
>  		.classzone_idx = classzone_idx,
>  		.direct_compaction = true,
> -		.whole_zone = (prio == COMPACT_PRIO_SYNC_FULL),
> -		.ignore_skip_hint = (prio == COMPACT_PRIO_SYNC_FULL)
> +		.whole_zone = (prio == MIN_COMPACT_PRIORITY),
> +		.ignore_skip_hint = (prio == MIN_COMPACT_PRIORITY)
>  	};
>  	INIT_LIST_HEAD(&cc.freepages);
>  	INIT_LIST_HEAD(&cc.migratepages);
> @@ -1691,7 +1691,7 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
>  								ac->nodemask) {
>  		enum compact_result status;
>  
> -		if (prio > COMPACT_PRIO_SYNC_FULL
> +		if (prio > MIN_COMPACT_PRIORITY
>  					&& compaction_deferred(zone, order)) {
>  			rc = max_t(enum compact_result, COMPACT_DEFERRED, rc);
>  			continue;
> -- 
> 2.9.2
> 
> 

-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>