Re: [PATCH v2] sched: print parent comm in sched_show_task()

[Petr Mladek <pmladek@suse.com>]

On Thu 2023-01-19 19:06:42, 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.
> Also, we move the parent info to a following new line while keeping the
> original line the same.
> And we would print "parent:unknown         ppid:<NULL>"
> when the task is not alive.
> 
> Signed-off-by: Tio Zhang <tiozhang@didiglobal.com>
> ---
>  kernel/sched/core.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index cb2aa2b54c7a..5690a5700f9e 100644
> --- 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 = NULL;
>  
>  	if (!try_get_task_stack(p))
>  		return;
> @@ -8867,13 +8868,22 @@ void sched_show_task(struct task_struct *p)
>  #endif
>  	ppid = 0;
>  	rcu_read_lock();
> -	if (pid_alive(p))
> -		ppid = task_pid_nr(rcu_dereference(p->real_parent));
> +	if (pid_alive(p)) {
> +		struct task_struct *parent = rcu_dereference(p->real_parent);
> +
> +		ppid = task_pid_nr(parent);
> +		pcomm = parent->comm;
> +	}
>  	rcu_read_unlock();
>  	pr_cont(" stack:%-5lu pid:%-5d ppid:%-6d flags:0x%08lx\n",
>  		free, task_pid_nr(p), ppid,

This prints "ppid:0" when pid_alive() returns false.

>  		read_task_thread_flags(p));
>  
> +	if (!ppid)
> +		pr_info("parent:unknown         ppid:<NULL>\n");

This prints "ppid:<NULL>". The inconsistency is weird.

Also there is no need to print ppid twice. I would remove it
from the " stack:..." line as it was done in the previous version
of the patch.

> +	else

> +		pr_info("parent:%-15.15s ppid:%-6d\n", pcomm, ppid);

"pcomm" must not be accessed after rcu_read_unlock(). The parent might
disappear in the meantime.

I suggest to got back to the previous version and just add printing
"parent:unknown         ppid:<NULL>\n" when pid_alive(p) fails.

Best Regards,
Petr