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Sender: Tejun Heo <htejun@gmail.com>
Date:   Fri, 12 Aug 2022 09:06:14 -1000
From:   Tejun Heo <tj@kernel.org>
To:     "zhaoyang.huang" <zhaoyang.huang@unisoc.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Zhaoyang Huang <huangzhaoyang@gmail.com>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        ke.wang@unisoc.com, Zefan Li <lizefan.x@bytedance.com>
Subject: Re: [RFC PATCH] cgroup: use root_mem_cgroup as css when current is
 not enabled
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On Fri, Aug 12, 2022 at 06:09:26PM +0800, zhaoyang.huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> 
> Memory charged on group B abserved on belowing v2 hierarchy where we just would
> like to only have group E's memory be controlled and B's descendants compete freely
> for memory. This should be the consequences of unified hierarchy. Solve this by
> have the cgroup without valid memory css alloced use root_mem_cgroup instead of
> its ancestor's.
> 
>  A(subtree_control = memory) - B(subtree_control = NULL) - C()
>                                                          \ D()
> 			     - E(subtree_control = memory) - F()
> 							   \ G()
> 
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> ---
>  kernel/cgroup/cgroup.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 1779ccd..b29b3f6 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -533,6 +533,14 @@ static struct cgroup_subsys_state *cgroup_e_css_by_mask(struct cgroup *cgrp,
>  	 * can't test the csses directly.  Test ss_mask.
>  	 */
>  	while (!(cgroup_ss_mask(cgrp) & (1 << ss->id))) {
> +		/*
> +		 * charging to the parent cgroup which hasn't distribute
> +		 * memory control to its descendants doesn't make sense
> +		 * especially on cgroup v2, where the parent could be configured
> +		 * to use memory controller as its sibling want to use it
> +		 */
> +		if (memory_cgrp_id == ss->id)
> +			return &root_mem_cgroup->css;

This is gonna be a hard nack. A given cgroup always encompasses all the
resources consumed in its self-including subtree.

Thanks.

-- 
tejun