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Date:   Wed, 11 Mar 2020 12:31:53 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Ivan Teterevkov <ivan.teterevkov@nutanix.com>
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Subject: Re: [PATCH] mm/vmscan: add vm_swappiness configuration knobs
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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.