Re: [PATCH] mm, slab: Extend slab/shrink to shrink all the memcg caches

[Andrew Morton]

On Tue,  2 Jul 2019 14:37:30 -0400 Waiman Long <longman@xxxxxxxxxx> wrote:

> Currently, a value of '1" is written to /sys/kernel/slab/<slab>/shrink
> file to shrink the slab by flushing all the per-cpu slabs and free
> slabs in partial lists. This applies only to the root caches, though.
> 
> Extends this capability by shrinking all the child memcg caches and
> the root cache when a value of '2' is written to the shrink sysfs file.

Why?

Please fully describe the value of the proposed feature to or users. 
Always.

> 
> ...
>
> --- a/Documentation/ABI/testing/sysfs-kernel-slab
> +++ b/Documentation/ABI/testing/sysfs-kernel-slab
> @@ -429,10 +429,12 @@ KernelVersion:	2.6.22
>  Contact:	Pekka Enberg <penberg@xxxxxxxxxxxxxx>,
>  		Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
>  Description:
> -		The shrink file is written when memory should be reclaimed from
> -		a cache.  Empty partial slabs are freed and the partial list is
> -		sorted so the slabs with the fewest available objects are used
> -		first.
> +		A value of '1' is written to the shrink file when memory should
> +		be reclaimed from a cache.  Empty partial slabs are freed and
> +		the partial list is sorted so the slabs with the fewest
> +		available objects are used first.  When a value of '2' is
> +		written, all the corresponding child memory cgroup caches
> +		should be shrunk as well.  All other values are invalid.

One would expect this to be a bitfield, like /proc/sys/vm/drop_caches. 
So writing 3 does both forms of shrinking.

Yes, it happens to be the case that 2 is a superset of 1, but what
about if we add "4"?