linux-kernel - Re: [PATCH] sched: print parent comm in sched_show

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Date:   Wed, 4 Jan 2023 14:24:11 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        zyhtheonly@...il.com, zyhtheonly@...h.net, zwp10758@...il.com
Subject: Re: [PATCH] sched: print parent comm in sched_show_task()

On Wed 2022-12-28 00:14:00, Tio Zhang wrote:
> Knowing who the parent is might be useful for debugging.
> For example, we can sometimes resolve kernel hung tasks by stopping
> the person who begins those hung tasks.
> With the parent's name printed in sched_show_task(),
> it might be helpful to let people know which "service" should be operated.

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8854,6 +8854,7 @@ void sched_show_task(struct task_struct *p)
>  {
>  	unsigned long free = 0;
>  	int ppid;
> +	char pcomm[TASK_COMM_LEN];
>  
>  	if (!try_get_task_stack(p))
>  		return;
> @@ -8867,11 +8868,13 @@ void sched_show_task(struct task_struct *p)
>  #endif
>  	ppid = 0;

We need to intialized pcomm here:

	pcomm[0] = '\0';

Otherwise, it would include a garbage when pid_alive(p) returns false below..

>  	rcu_read_lock();
> -	if (pid_alive(p))
> +	if (pid_alive(p)) {
>  		ppid = task_pid_nr(rcu_dereference(p->real_parent));
> +		get_task_comm(pcomm, rcu_dereference(p->real_parent));
> +	}
>  	rcu_read_unlock();
> -	pr_cont(" stack:%-5lu pid:%-5d ppid:%-6d flags:0x%08lx\n",
> -		free, task_pid_nr(p), ppid,
> +	pr_cont(" stack:%-5lu pid:%-5d ppid:%-6d parent:%-15.15s
> flags:0x%08lx\n",

It would print:  .... parent:xxx flags:0000

Some people might be confused whether the flags are from
the task or from the parent.

A solution would be to move the parent value to another line.
It would even better solve the situation when the task
is not alive and we could not get information about the parent:

	if (pid_alive(p)) {
		struct parent = rcu_dereference(p->real_parent);

		pr_info("parent:%-15.15s ppid:%-6d\n",
			parent->comm, task_pid_nr(parent));
	}

> +		free, task_pid_nr(p), ppid, pcomm,
>  		read_task_thread_flags(p));
>  
>  	print_worker_info(KERN_INFO, p);

Best Regards,
Petr