Comment 36 for bug 1861359

OK, so we know that it's not related to the memory cgroup subsystem.

Another reason of such unexpected swapping activity could be due to memory compaction code that is triggering some direct memory reclaim and forcing to swap out pages.

What do you have in /proc/sys/vm/swappiness? Could you try to set that to 0 (kernel prefers to drop file-backed pages instead of swapping out anonymous pages) and see if the swap out activity is still happening?

Moreover, we can do some more advanced tracing of the direct memory reclaim code, for example:
 $ sudo kprobe-perf -s 'p:shrink_node'

This may give us useful information if and how direct memory reclaim code was triggered (this command should print out some stack traces when notice the swap out activity).