linux-kernel - Re: [PATCH] x86/resctrl: Only show tasks' pids in current pid namespace

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Date:   Wed, 15 Feb 2023 13:43:28 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Shawn Wang <shawnwang@...ux.alibaba.com>, <fenghua.yu@...el.com>
CC:     <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
        <dave.hansen@...ux.intel.com>, <james.morse@....com>,
        <x86@...nel.org>, <hpa@...or.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/resctrl: Only show tasks' pids in current pid
 namespace

Hi Shawn,

On 1/15/2023 11:12 PM, Shawn Wang wrote:
> When writing a task id to the "tasks" file in an rdtgroup,
> rdtgroup_tasks_write() treats the pid as a number in the current pid
> namespace. But when reading the "tasks" file, rdtgroup_tasks_show() shows
> the list of global pids from the init namespace. If current pid namespace
> is not the init namespace, pids in "tasks" will be confusing and incorrect.
> 
> To be more robust, let the "tasks" file only show pids in the current pid
> namespace.
> 

Is it possible to elaborate more on the use case that this is aiming to
address? It is unexpected to me that resource management is approached from
within a container. My expectation is that the resource management and monitoring
is done from the host. 

> Signed-off-by: Shawn Wang <shawnwang@...ux.alibaba.com>
> ---
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 5993da21d822..9e97ae24c159 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -718,11 +718,15 @@ static ssize_t rdtgroup_tasks_write(struct kernfs_open_file *of,
>  static void show_rdt_tasks(struct rdtgroup *r, struct seq_file *s)
>  {
>  	struct task_struct *p, *t;
> +	pid_t pid;
>  
>  	rcu_read_lock();
>  	for_each_process_thread(p, t) {
> -		if (is_closid_match(t, r) || is_rmid_match(t, r))
> -			seq_printf(s, "%d\n", t->pid);
> +		if (is_closid_match(t, r) || is_rmid_match(t, r)) {
> +			pid = task_pid_vnr(t);
> +			if (pid)
> +				seq_printf(s, "%d\n", pid);
> +		}
>  	}
>  	rcu_read_unlock();
>  }

This looks like it would solve the stated problem. Does it slow down
reading a tasks file in a measurable way?

Reinette