Re: [PATCH] mm/memcontrol: exclude @root from checks in mem_cgroup_low

[Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>]

On Fri, Jun 02, 2017 at 01:13:32PM -0700, Sean Christopherson wrote:
> Make @root exclusive in mem_cgroup_low; it is never considered low
> when looked at directly and is not checked when traversing the tree.
> In effect, @root is handled identically to how root_mem_cgroup was
> previously handled by mem_cgroup_low.
> 
> If @root is not excluded from the checks, a cgroup underneath @root
> will never be considered low during targeted reclaim of @root, e.g.
> due to memory.current > memory.high, unless @root is misconfigured
> to have memory.low > memory.high.
> 
> Excluding @root enables using memory.low to prioritize memory usage
> between cgroups within a subtree of the hierarchy that is limited by
> memory.high or memory.max, e.g. when ROOT owns @root's controls but
> delegates the @root directory to a USER so that USER can create and
> administer children of @root.
> 
> For example, given cgroup A with children B and C:
> 
>     A
>    / \
>   B   C
> 
> and
> 
>   1. A/memory.current > A/memory.high
>   2. A/B/memory.current < A/B/memory.low
>   3. A/C/memory.current >= A/C/memory.low
> 
> As 'A' is high, i.e. triggers reclaim from 'A', and 'B' is low, we
> should reclaim from 'C' until 'A' is no longer high or until we can
> no longer reclaim from 'C'.  If 'A', i.e. @root, isn't excluded by
> mem_cgroup_low when reclaming from 'A', then 'B' won't be considered
> low and we will reclaim indiscriminately from both 'B' and 'C'.
> 
> Signed-off-by: Sean Christopherson <sean.j.christopherson-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>

Good catch, thank you Sean.

Acked-by: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>