** Changed in: linux (Ubuntu Oracular)
Status: In Progress => Fix Committed
** Changed in: linux (Ubuntu Noble)
Status: In Progress => Fix Committed
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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 Committed
Status in linux source package in Oracular:
Fix Committed
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 <[email protected]>
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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