linux-kernel - Re: Let's talk about the elephant in the room - the Linux kernel's inability to gracefully handle low memory pressure

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Date:   Fri, 9 Aug 2019 16:56:28 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Artem S. Tashkinov" <aros@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: Let's talk about the elephant in the room - the Linux kernel's
 inability to gracefully handle low memory pressure

On 8/8/19 7:27 PM, Johannes Weiner wrote:
> On Thu, Aug 08, 2019 at 04:47:18PM +0200, Vlastimil Babka wrote:
>> On 8/7/19 10:51 PM, Johannes Weiner wrote:
>>> From 9efda85451062dea4ea287a886e515efefeb1545 Mon Sep 17 00:00:00 2001
>>> From: Johannes Weiner <hannes@...xchg.org>
>>> Date: Mon, 5 Aug 2019 13:15:16 -0400
>>> Subject: [PATCH] psi: trigger the OOM killer on severe thrashing
>>
>> Thanks a lot, perhaps finally we are going to eat the elephant ;)
>>
>> I've tested this by booting with mem=8G and activating browser tabs as
>> long as I could. Then initially the system started thrashing and didn't
>> recover for minutes. Then I realized sysrq+f is disabled... Fixed that
>> up after next reboot, tried lower thresholds, also started monitoring
>> /proc/pressure/memory, and found out that after minutes of not being
>> able to move the cursor, both avg10 and avg60 shows only around 15 for
>> both some and full. Lowered thrashing_oom_level to 10 and (with
>> thrashing_oom_period of 5) the thrashing OOM finally started kicking,
>> and the system recovered by itself in reasonable time.
> 
> It sounds like there is a missing annotation. The time has to be going
> somewhere, after all. One *known* missing vector I fixed recently is
> stalls in submit_bio() itself when refaulting, but it's not merged
> yet. Attaching the patch below, can you please test it?

It made a difference, but not enough, it seems. Before the patch I could
observe "io:full avg10" around 75% and "memory:full avg10" around 20%,
after the patch, "memory:full avg10" went to around 45%, while io stayed
the same (BTW should the refaults be discounted from the io counters, so
that the sum is still <=100%?)
As a result I could change the knobs to recover successfully with
thrashing detected for 10s of 40% memory pressure.

Perhaps being low on memory we can't detect refaults so well due to
limited number of shadow entries, or there was genuine non-refault I/O
in the mix. The detection would then probably have to look at both I/O
and memory?

Thanks,
Vlastimil