[PATCH 6/6] sched/isolation: Enable 'remote_pcpu_cache_access' on NOHZ_FULL systems

[Nicolas Saenz Julienne]

When enabled, 'remote_pcpu_cache_access' allows for remote draining of
mm/swap.c's per-cpu LRU caches and mm/page_alloc.c's per-cpu page lists
instead of using per-cpu drain work. This comes at the cost of more
constraining locking, but NOHZ_FULL setups need this nonetheless:
processes running on isolated CPUs are sensitive to any sort of
interruption and preempting them in order to satisfy a housekeeping task
is bound to break their functional guarantees (i.e. latency, bandwidth,
etc...).

So enable 'remote_pcpu_cache_access' after having successfully
initialized NOHZ_FULL.

This is based on previous work by Thomas Gleixner, Anna-Maria Gleixner,
and Sebastian Andrzej Siewior[1].

[1] https://patchwork.kernel.org/project/linux-mm/patch/20190424111208.24459-3-bigeasy@xxxxxxxxxxxxx/
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>
---
 kernel/sched/isolation.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 7f06eaf12818..4fc4c76f27ab 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -8,6 +8,7 @@
  *
  */
 #include "sched.h"
+#include "../../mm/internal.h"
 
 DEFINE_STATIC_KEY_FALSE(housekeeping_overridden);
 EXPORT_SYMBOL_GPL(housekeeping_overridden);
@@ -137,11 +138,17 @@ static int __init housekeeping_setup(char *str, enum hk_flags flags)
 static int __init housekeeping_nohz_full_setup(char *str)
 {
 	unsigned int flags;
+	int ret;
 
 	flags = HK_FLAG_TICK | HK_FLAG_WQ | HK_FLAG_TIMER | HK_FLAG_RCU |
 		HK_FLAG_MISC | HK_FLAG_KTHREAD;
 
-	return housekeeping_setup(str, flags);
+	ret = housekeeping_setup(str, flags);
+	if (ret)
+		/* Avoid LRU cache and mm/page_alloc.c's pcplists drain work */
+		static_branch_enable(&remote_pcpu_cache_access);
+
+	return ret;
 }
 __setup("nohz_full=", housekeeping_nohz_full_setup);
 
-- 
2.31.1