[PATCH] mm/vmscan: add vm_swappiness configuration knobs

[Ivan Teterevkov]

This patch adds a couple of knobs:

- The configuration option (CONFIG_VM_SWAPPINESS).
- The command line parameter (vm_swappiness).

The default value is preserved, but now defined by CONFIG_VM_SWAPPINESS.

Historically, the default swappiness is set to the well-known value 60,
and this works well for the majority of cases. The vm_swappiness is also
exposed as the kernel parameter that can be changed at runtime too, e.g.
with sysctl.

This approach might not suit well some configurations, e.g. systemd-based
distros, where systemd is put in charge of the cgroup controllers,
including the memory one. In such cases, the default swappiness 60
is copied across the cgroup subtrees early at startup, when systemd
is arranging the slices for its services, before the sysctl.conf
or tmpfiles.d/*.conf changes are applied.

One could run a script to traverse the cgroup trees later and set the
desired memory.swappiness individually in each occurrence when the runtime
is set up, but this would require some amount of work to implement
properly. Instead, why not set the default swappiness as early as possible?

Signed-off-by: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |  4 ++++
 mm/Kconfig                                    | 10 ++++++++
 mm/vmscan.c                                   | 24 ++++++++++++++++++-
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index c07815d230bc..5d54a4303522 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5317,6 +5317,10 @@
 			  P	Enable page structure init time poisoning
 			  -	Disable all of the above options
 
+	vm_swappiness=	[KNL]
+			Sets the default vm_swappiness.
+			Ranges from 0 to 100, the default value is 60.
+
 	vmalloc=nn[KMG]	[KNL,BOOT] Forces the vmalloc area to have an exact
 			size of <nn>. This can be used to increase the
 			minimum size (128MB on x86). It can also be used to diff --git a/mm/Kconfig b/mm/Kconfig index ab80933be65f..ec59c19e578e 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -739,4 +739,14 @@ config ARCH_HAS_HUGEPD  config MAPPING_DIRTY_HELPERS
         bool
 
+config VM_SWAPPINESS
+	int "Default memory swappiness"
+	default 60
+	range 0 100
+	help
+	  Sets the default vm_swappiness, that could be changed later
+	  in the runtime, e.g. kernel command line, sysctl, etc.
+
+	  Higher value means more swappy. Historically, defaults to 60.
+
 endmenu
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 876370565455..7d2d3550f698 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -163,7 +163,29 @@ struct scan_control {
 /*
  * From 0 .. 100.  Higher means more swappy.
  */
-int vm_swappiness = 60;
+int vm_swappiness = CONFIG_VM_SWAPPINESS;
+
+static int __init swappiness_cmdline(char *str) {
+	int val, err;
+
+	if (!str)
+		return -EINVAL;
+
+	err = kstrtoint(str, 10, &val);
+	if (err)
+		return -EINVAL;
+
+	if (val < 0 || val > 100)
+		return -EINVAL;
+
+	vm_swappiness = val;
+
+	return 0;
+}
+
+early_param("vm_swappiness", swappiness_cmdline);
+
 /*
  * The total number of pages which are beyond the high watermark within all
  * zones.
--
2.25.0