Re: OOM detection regressions since 4.7

On Mon, Aug 22, 2016 at 11:32:49AM +0200, Michal Hocko wrote:

Hi, 
there have been multiple reports [1][2][3][4][5] about pre-mature OOM
killer invocations since 4.7 which contains oom detection rework. All of
them were for order-2 (kernel stack) alloaction requests failing because
of a high fragmentation and compaction failing to make any forward
progress. While investigating this we have found out that the compaction
just gives up too early. Vlastimil has been working on compaction
improvement for quite some time and his series [6] is already sitting
in mmotm tree. This already helps a lot because it drops some heuristics
which are more aimed at lower latencies for high orders rather than
reliability. Joonsoo has then identified further problem with too many
blocks being marked as unmovable [7] and Vlastimil has prepared a patch
on top of his series [8] which is also in the mmotm tree now.

That being said, the regression is real and should be fixed for 4.7
stable users. [6][8] was reported to help and ooms are no longer
reproducible. I know we are quite late (rc3) in 4.8 but I would vote
for mergeing those patches and have them in 4.8. For 4.7 I would go
with a partial revert of the detection rework for high order requests
(see patch below). This patch is really trivial. If those compaction
improvements are just too large for 4.8 then we can use the same patch
as for 4.7 stable for now and revert it in 4.9 after compaction changes
are merged.

Thoughts?

[1] http://lkml.kernel.org/r/20160731051121.GB307@x4
[2] http://lkml.kernel.org/r/201608120901.41463.a.miskiewicz@gmail.com
[3] http://lkml.kernel.org/r/20160801192620.GD31957@dhcp22.suse.cz
[4] https://lists.opensuse.org/opensuse-kernel/2016-08/msg00021.html
[5] https://bugzilla.opensuse.org/show_bug.cgi?id=994066
[6] http://lkml.kernel.org/r/20160810091226.6709-1-vbabka@suse.cz
[7] http://lkml.kernel.org/r/20160816031222.GC16913@js1304-P5Q-DELUXE
[8] http://lkml.kernel.org/r/f7a9ea9d-bb88-bfd6-e340-3a933559305a@suse.cz

---

quoted

From 899b738538de41295839dca2090a774bdd17acd2 Mon Sep 17 00:00:00 2001

From: Michal Hocko <mhocko@suse.com>
Date: Mon, 22 Aug 2016 10:52:06 +0200
Subject: [PATCH] mm, oom: prevent pre-mature OOM killer invocation for high
 order request

There have been several reports about pre-mature OOM killer invocation
in 4.7 kernel when order-2 allocation request (for the kernel stack)
invoked OOM killer even during basic workloads (light IO or even kernel
compile on some filesystems). In all reported cases the memory is
fragmented and there are no order-2+ pages available. There is usually
a large amount of slab memory (usually dentries/inodes) and further
debugging has shown that there are way too many unmovable blocks which
are skipped during the compaction. Multiple reporters have confirmed that
the current linux-next which includes [1] and [2] helped and OOMs are
not reproducible anymore. A simpler fix for the stable is to simply
ignore the compaction feedback and retry as long as there is a reclaim
progress for high order requests which we used to do before. We already
do that for CONFING_COMPACTION=n so let's reuse the same code when
compaction is enabled as well.

Hello, Michal.

I agree with partial revert but revert should be a different form.
Below change try to reuse should_compact_retry() version for
!CONFIG_COMPACTION but it turned out that it also causes regression in
Markus report [1].

Theoretical reason for this regression is that it would stop retry
even if there are enough lru pages. It only checks if freepage
excesses min watermark or not for retry decision. To prevent
pre-mature OOM killer, we need to keep allocation loop when there are
enough lru pages. So, logic should be something like that.

should_compact_retry()
{
        for_each_zone_zonelist_nodemask {
                available = zone_reclaimable_pages(zone);
                available += zone_page_state_snapshot(zone, NR_FREE_PAGES);
                if (__zone_watermark_ok(zone, *0*, min_wmark_pages(zone),
                        ac_classzone_idx(ac), alloc_flags, available))
                        return true;

        }
}

I suggested it before and current situation looks like it is indeed
needed.

And, I still think that your OOM detection rework has some flaws.

1) It doesn't consider freeable objects that can be freed by shrink_slab().
There are many subsystems that cache many objects and they will be
freed by shrink_slab() interface. But, you don't account them when
making the OOM decision.

Think about following situation that we are trying to find order-2
freepage and some subsystem has order-2 freepage. It can be freed by
shrink_slab(). Your logic doesn't guarantee that shrink_slab() is
invoked to free this order-2 freepage in that subsystem. OOM would be
triggered when compaction fails even if there is a order-2 freeable
page. I think that if decision is made before whole lru list is
scanned and then shrink_slab() is invoked for whole freeable objects,
it would cause pre-mature OOM.

It seems that you already knows this issue [2].

2) 'OOM detection rework' depends on compaction too much. Compaction
algorithm is racy and has some limitation. It's failure doesn't mean we
are in OOM situation. Even if Vlastimil's patchset and mine is
applied, it is still possible that compaction scanner cannot find enough
freepage due to race condition and return pre-mature failure. To
reduce this race effect, I hope to give more chances to retry even if
full compaction is failed. We can remove this heuristic when we make
sure that compaction is stable enough.

As you know, I said these things several times but isn't accepted.
Please consider them more deeply at this time.

Thanks.

[1] http://lkml.kernel.org/r/20160731051121.GB307@x4
[2] https://bugzilla.opensuse.org/show_bug.cgi?id=994066

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