Re: Minor page faults from memory compaction causing in-band transitions

[Philippe Gerum <rpm@xenomai.org>]

Brandon Ho <brandonho667@gmail.com> writes:

> Hi Xenomai team,
>
> I'm working on a real-time control application using Xenomai and we've been
> experiencing unexpected in-band transitions caused by minor page faults during
> kernel memory compaction. Even though our RT threads use mlockall(MCL_CURRENT |
> MCL_FUTURE) and we've pre-faulted memory, the kernel's compaction process seems
> to temporarily invalidate PTEs on our locked pages, causing faults when the RT
> thread accesses them.
>
> Has this issue come up before?

Yes, this is a known issue.

> I'm wondering if there's a mechanism to reserve
> a section of RAM that's excluded from compaction/migration entirely,
> or if

Not to my knowledge.

> there are kernel/Xenomai configurations we should be using to prevent this.
>
> Any guidance would be appreciated!
>

As Jan mentioned, the only way is to disable all options which select
CONFIG_COMPACTION ATM.

-- 
Philippe.