[PATCH] mm/compaction: Disable compact_unevictable_allowed on RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

Since commit
    5bbe3547aa3ba ("mm: allow compaction of unevictable pages")

it is allowed to examine mlocked pages and compact them by default.
On -RT even minor pagefaults are problematic because it may take a few
100us to resolve them and until then the task is blocked.

Make compact_unevictable_allowed = 0 default and remove it from /proc on
RT.

Link: https://lore.kernel.org/linux-mm/20190710144138.qyn4tuttdq6h7kqx@linutronix.de/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/sysctl.c | 3 ++-
 mm/compaction.c | 4 ++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 70665934d53e2..d08bd51a0fbc3 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1488,6 +1488,7 @@ static struct ctl_table vm_table[] = {
 		.extra1		= &min_extfrag_threshold,
 		.extra2		= &max_extfrag_threshold,
 	},
+#ifndef CONFIG_PREEMPT_RT
 	{
 		.procname	= "compact_unevictable_allowed",
 		.data		= &sysctl_compact_unevictable_allowed,
@@ -1497,7 +1498,7 @@ static struct ctl_table vm_table[] = {
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_ONE,
 	},
-
+#endif
 #endif /* CONFIG_COMPACTION */
 	{
 		.procname	= "min_free_kbytes",
diff --git a/mm/compaction.c b/mm/compaction.c
index 672d3c78c6abf..b2c804c35ae56 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1590,7 +1590,11 @@ typedef enum {
  * Allow userspace to control policy on scanning the unevictable LRU for
  * compactable pages.
  */
+#ifdef CONFIG_PREEMPT_RT
+#define sysctl_compact_unevictable_allowed 0
+#else
 int sysctl_compact_unevictable_allowed __read_mostly = 1;
+#endif
 
 static inline void
 update_fast_start_pfn(struct compact_control *cc, unsigned long pfn)
-- 
2.25.0.rc2