linux-kernel - Re: [RFC 2/5] workqueue: Warn when a new worker could not be created

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Date:   Thu, 16 Feb 2023 10:43:13 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC 2/5] workqueue: Warn when a new worker could not be created

On Wed 2023-02-15 19:02:04, Michal Koutný wrote:
> Hello.
> 
> On Wed, Feb 01, 2023 at 02:45:40PM +0100, Petr Mladek <pmladek@...e.com> wrote:
> >    + the system reached PID limit
> or threads-max limit.
> 
> FTR, I was once considering something like
> 
> --->8---
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 867b46d6fd0a..bba05ecc3765 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1684,8 +1684,10 @@ static __latent_entropy struct task_struct *copy_process(
>          * to stop root fork bombs.
>          */
>         retval = -EAGAIN;
> -       if (nr_threads >= max_threads)
> +       if (nr_threads >= max_threads) {
> +               printk_once(KERN_INFO "clone failed due to threads-max limit\n");
>                 goto bad_fork_cleanup_count;
> +       }
> 
>         delayacct_tsk_init(p);  /* Must remain after dup_task_struct() */
>         p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE);
> @@ -1816,6 +1818,7 @@ static __latent_entropy struct task_struct *copy_process(
>         if (pid != &init_struct_pid) {
>                 pid = alloc_pid(p->nsproxy->pid_ns_for_children);
>                 if (IS_ERR(pid)) {
> +                       printk_once(KERN_INFO "fork failed to find pid\n");
>                         retval = PTR_ERR(pid);
>                         goto bad_fork_cleanup_thread;
>                 }
> --->8---
> 
> Effects of the global limits on anything but kthreads should be much less
> important and easier to troubleshoot anyway.
> Covering kworkers with your changes should be useful and substitute my idea
> above.

I was not brave enough to touch kernel/fork.c.

> Take that as my support for this patch (from my perspective reporting
> *_once would be enough to guide a troubleshooter).

My experience is that users often send logs with many stall reports and
the earlier messages are missing/lost. This is why I wanted to repeat
them.

Anyway, thanks for the support. I am going to send v2 when time permits.

Best Regards,
Petr