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SubjectRe: [PATCH] mm/vmscan: add vm_swappiness configuration knobs
On Wed, 11 Mar 2020, Ivan Teterevkov wrote:

> 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.
>

Seems like something that can be fully handled by an initscript that would
set the sysctl and then iterate the memcg hierarchy propagating the
non-default value. I don't think that's too much of an ask if userspace
wants to manipulate the swappiness value.

Or maybe we can be more clever: have memcg->swappiness store -1 by default
unless it is changed by the user explicitly and then have
mem_cgroup_swappiness() return vm_swappiness for this value. If the user
overwrites it, it's intended.

So there are a couple options here but I don't think one of them is to add
a new config option or kernel command line option.

> 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@nutanix.com>
> ---
> .../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.

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