Re: Realtime threads delayed due to kcompactd0

[Alexander Krabler <Alexander.Krabler@kuka.com>]

On Thu, 7 Aug 2025 14:21, Hugh Dickins wrote:
> But another idea has just this moment occurred to me: anon THP splitting
> is another user of migration entries.  Is it possible that kcompactd is
> not actually the cause of Alexander's issue, but that his RT tasks have
> (bad news! shouldn't be allowed) got anon THPs in them?

No, we don't have transparent hugepages enabled.
(From Kconfig, this seems not even possible together with PREEMPT_RT.)

From ftrace events (first message in this thread), we know that kcompactd0
finally wakes our realtime thread(s).

Given the information I got from here, the comment on the code [1]
and an older commit message [2], I suspect CMA somehow influences our problem.

If there is anything we should enable for better diagnosis (e.g. more ftrace events),
we would be willing to try that out.

[1] https://elixir.bootlin.com/linux/v6.16/source/mm/compaction.c#L1136
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e46a28790e594c0876d1a84270926abf75460f61

Thanks,
Alexander

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