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

[Philippe Gerum <rpm@xenomai.org>]

Philippe Gerum <rpm@xenomai.org> writes:

> Florian Bezdeka <florian.bezdeka@siemens.com> writes:
>
>> On Mon, 2026-04-13 at 20:31 +0200, Philippe Gerum wrote:
>>> Brandon Ho <brandonho667@gmail.com> writes:
>>> 
>>> > Thank you for the responses! I recently came across the
>>> > `compact_unevictable_allowed` sysctl option and it seems like it could be a
>>> > good fit for our use case. Setting it to 0 would prevent the kernel from
>>> > compacting/migrating locked pages, which should eliminate the minor faults
>>> > we're seeing during compaction.
>>> > 
>>> > Could we possibly set `compact_unevictable_allowed=0` by default when
>>> > CONFIG_EVL is enabled?
>>> 
>>> Definitely, yes. Thanks for chasing this issue down. This patch is going
>>> to be merged upstream asap, you may want to apply it for a preview.
>>
>> Targeting linux-evl or linux-dovetail?
>>
>
> linux-dovetail, this is a general issue for any latency-sensitive code.

While reviewing all PREEMPT_RT exclusion cases in Kconfigs, some of
which Dovetail may want to share, I found this potential one:

--- a/init/Kconfig
+++ b/init/Kconfig
@@ -947,7 +947,7 @@ config NUMA_BALANCING
 	bool "Memory placement aware NUMA scheduler"
 	depends on ARCH_SUPPORTS_NUMA_BALANCING
 	depends on !ARCH_WANT_NUMA_VARIABLE_LOCALITY
-	depends on SMP && NUMA && MIGRATION && !PREEMPT_RT
+	depends on SMP && NUMA && MIGRATION && !PREEMPT_RT && !DOVETAIL

Any objection to disabling NUMA_BALANCING when operating in dual kernel
mode? (CONFIG_DOVETAIL=y).

-- 
Philippe.