[Bug 2087886] Re: MGLRU: kswapd uses 100% CPU when MGLRU is enabled and under memory pressure

This bug is awaiting verification that the linux-
realtime-6.11/6.11.0-1003.3~24.04.2 kernel in -proposed solves the
problem. Please test the kernel and update this bug with the results. If
the problem is solved, change the tag 'verification-needed-noble-linux-
realtime-6.11' to 'verification-done-noble-linux-realtime-6.11'. If the
problem still exists, change the tag 'verification-needed-noble-linux-
realtime-6.11' to 'verification-failed-noble-linux-realtime-6.11'.


If verification is not done by 5 working days from today, this fix will
be dropped from the source code, and this bug will be closed.


See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how
to enable and use -proposed. Thank you!


** Tags added: kernel-spammed-noble-linux-realtime-6.11-v2 verification-needed-noble-linux-realtime-6.11

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https://bugs.launchpad.net/bugs/2087886

Title:
MGLRU: kswapd uses 100% CPU when MGLRU is enabled and under memory
pressure

Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Noble:
Fix Released
Status in linux source package in Oracular:
Fix Released

Bug description:
BugLink: https://bugs.launchpad.net/bugs/2087886

[Impact]

On systems with MGLRU enabled, which it is by default, if the system is under
memory pressure, and some pages are then allocated, such that it wakes up kswapd
to attempt page reclaim, but the system has enough memory that kswapd doesn't
get OOM killed, and then there are no pages to actually reclaim, kswapd can spin
at 100% endlessly, causing severe performance issues.

What's happening is that lru_gen_shrink_node() unconditionally clears
kswapd_failures, which can prevent kswapd from sleeping and cause 100% kswapd
cpu usage even when kswapd repeatedly fails to make progress in reclaim.

A workaround is to disable MGLRU, and the issue does not occur.

[Fix]

The fix is to only clear kswap_failures in lru_gen_shrink_node() if reclaim
makes some progress, similar to shrink_node().

This was fixed in 6.12-rc4 by the commit:

commit b130ba4a6259f6b64d8af15e9e7ab1e912bcb7ad
Author: Wei Xu <weixugc@google.com>
Date: Mon Oct 14 22:12:11 2024 +0000
Subject: mm/mglru: only clear kswapd_failures if reclaimable
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b130ba4a6259f6b64d8af15e9e7ab1e912bcb7ad

Both Noble and Oracular need this fix.

[Testcase]

The systems with this issue are DPDK compute nodes running OpenStack Yoga on
Jammy. If you leave them for several days, they will hit this issue and their
kswap processes will go to 100% and never drop.

    PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
    846 root 20 0 0 0 0 R 100.0 0.0 1915:16 kswapd0
    846 root 20 0 0 0 0 R 100.0 0.0 1915:16 kswapd0
    846 root 20 0 0 0 0 R 100.0 0.0 1915:17 kswapd0
    846 root 20 0 0 0 0 R 99.0 0.0 1915:18 kswapd0

If you disable MGLRU however, they drop to 0% and sleep immediately.

There is a test kernel available in the following ppa:

https://launchpad.net/~mruffell/+archive/ubuntu/sf390959-test

If you install the test kernel, the system should remain stable, and kswapd will
not go to 100% cpu after some time.

[Where problems could occur]

We are adding a small requirement to clearing kswapd_failures, that we actually
reclaim some memory. If we do actually reclaim some memory, the behaviour is the
same as what we have now, we clear kswapd_failures and continue.

If we don't manage to reclaim any memory, we leave kswapd_failures as is, and
let kswapd sleep, to try again at a later time.

If a regression were to occur, it would affect MGLRU users, which is the default.
A regression would look like issues with kswapd memory reclaim, or lead to high
cpu usage with kswapd.

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