Re: Realtime threads delayed due to kcompactd0

[Vlastimil Babka <vbabka@suse.cz>]

On 8/7/25 17:49, Alexander Krabler wrote:
> 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.

However, kcompactd doesn't perform CMA allocations, only compaction, in a
mode that does not include ISOLATE_UNEVICTABLE. So this is weird.

> 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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