Re: [PATCH] psi: Inherit parent cgroup psi enable state

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Chuyi,

On 7/29/2024 9:11 AM, Chuyi Zhou wrote:
> Currently when a parent cgroup disables psi through cgroup.pressure, newly
> created child cgroups do not inherit the psi state of the parent cgroup.
> 
> This patch tries to solve this issue. When a child cgroup is created, it
> would inherit the psi enabled state of the parent in group_init().
> Once the enable state is found to be false in the css_populate_dir(), the
> {cpu, io, memory}.pressure files will be hidden using cgroup_file_show().
> 
> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
> ---
>   kernel/cgroup/cgroup.c | 21 +++++++++++++++++++--
>   kernel/sched/psi.c     |  4 ++--
>   2 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index c8e4b62b436a4..775fe528efcad 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -1719,6 +1719,24 @@ static void css_clear_dir(struct cgroup_subsys_state *css)
>   	}
>   }
>   
> +static int populate_psi_files(struct cgroup_subsys_state *css)
> +{
> +	struct cgroup *cgrp = css->cgroup;
> +	int ret, i;
> +
> +	ret = cgroup_addrm_files(css, cgrp, cgroup_psi_files, true);
> +	if (ret < 0)
> +		return ret;
> +
> +	if (cgrp->psi && !cgrp->psi->enabled) {
> +		for (i = 0; i < NR_PSI_RESOURCES; i++)
> +			cgroup_file_show(&cgrp->psi_files[i], 0);
> +	}
> +
> +	return ret;
> +}
> +
> +
>   /**
>    * css_populate_dir - create subsys files in a cgroup directory
>    * @css: target css
> @@ -1742,8 +1760,7 @@ static int css_populate_dir(struct cgroup_subsys_state *css)
>   				return ret;
>   
>   			if (cgroup_psi_enabled()) {
> -				ret = cgroup_addrm_files(css, cgrp,
> -							 cgroup_psi_files, true);
> +				ret = populate_psi_files(css);
>   				if (ret < 0) {
>   					cgroup_addrm_files(css, cgrp,
>   							   cgroup_base_files, false);
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 020d58967d4e8..d0aa17b368819 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -180,7 +180,7 @@ static void group_init(struct psi_group *group)
>   {
>   	int cpu;
>   
> -	group->enabled = true;
> +	group->enabled = group->parent ? group->parent->enabled : true;

Since this is only the init path, if the user later enables PSI
accounting for a parent, should it not re-evaluate it for the groups
down the hierarchy?

Looking at "cgroup_pressure_write()", I could not spot it calling
"css_populate_dir()". Should it not walk the hierarchy and do a
"cgroup_file_show()" considering the changes in you patch?

(P.S. I'm not too familiar with this piece of code so please do let me
  know if I missed something obvious)

>   	for_each_possible_cpu(cpu)
>   		seqcount_init(&per_cpu_ptr(group->pcpu, cpu)->seq);
>   	group->avg_last_update = sched_clock();
> @@ -1114,8 +1114,8 @@ int psi_cgroup_alloc(struct cgroup *cgroup)
>   		kfree(cgroup->psi);
>   		return -ENOMEM;
>   	}
> -	group_init(cgroup->psi);
>   	cgroup->psi->parent = cgroup_psi(cgroup_parent(cgroup));
> +	group_init(cgroup->psi);
>   	return 0;
>   }
>   

-- 
Thanks and Regards,
Prateek