Subject: [PATCH] mm: Disable NUMA_BALANCING_DEFAULT_ENABLED and TRANSPARENT_HUGEPAGE on PREEMPT_RT
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Thu, 28 Oct 2021 16:33:27 +0200

TRANSPARENT_HUGEPAGE:
There are potential non-deterministic delays to an RT thread if a critical
memory region is not THP-aligned and a non-RT buffer is located in the same
hugepage-aligned region. It's also possible for an unrelated thread to migrate
pages belonging to an RT task incurring unexpected page faults due to memory
defragmentation even if khugepaged is disabled.

Regular HUGEPAGEs are not affected by this can be used.

NUMA_BALANCING:
There is a non-deterministic delay to mark PTEs PROT_NONE to gather NUMA fault
samples, increased page faults of regions even if mlocked and non-deterministic
delays when migrating pages.

[Mel Gorman worded 99% of the commit description].

Link: https://lore.kernel.org/all/[email-masked]/
Link: https://lore.kernel.org/all/[email-masked]/
Cc: Mel Gorman <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Acked-by: Mel Gorman <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
---
 init/Kconfig |    2 +-
 mm/Kconfig   |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/init/Kconfig
+++ b/init/Kconfig
@@ -901,7 +901,7 @@ 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
+	depends on SMP && NUMA && MIGRATION && !PREEMPT_RT
 	help
 	  This option adds support for automatic NUMA aware memory/task placement.
 	  The mechanism is quite primitive and is based on migrating memory when
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -371,7 +371,7 @@ config NOMMU_INITIAL_TRIM_EXCESS
 
 config TRANSPARENT_HUGEPAGE
 	bool "Transparent Hugepage Support"
-	depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE
+	depends on HAVE_ARCH_TRANSPARENT_HUGEPAGE && !PREEMPT_RT
 	select COMPACTION
 	select XARRAY_MULTI
 	help