Re: [PATCH RFC] net: memcg accounting for veth devices

["Eric W. Biederman" <ebiederm@xmission.com>]

Luis Chamberlain <mcgrof@kernel.org> writes:

> On Tue, Mar 01, 2022 at 02:50:06PM -0600, Eric W. Biederman wrote:
>> I really have not looked at this pids controller.
>> 
>> So I am not certain I understand your example here but I hope I have
>> answered your question.
>
> During experimentation with the above stress-ng test case, I saw tons
> of thread just waiting to do exit:

You increment the count of concurrent threads after a no return function
in do_exit.  Since the increment is never reached the count always goes
down and eventually the warning prints.

> diff --git a/kernel/exit.c b/kernel/exit.c
> index 80c4a67d2770..653ca7ebfb58 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -730,11 +730,24 @@ static void check_stack_usage(void)
>  static inline void check_stack_usage(void) {}
>  #endif
>  
> +/* Approx more than twice max_threads */
> +#define MAX_EXIT_CONCURRENT (1<<17)
> +static atomic_t exit_concurrent_max = ATOMIC_INIT(MAX_EXIT_CONCURRENT);
> +static DECLARE_WAIT_QUEUE_HEAD(exit_wq);
> +
>  void __noreturn do_exit(long code)
>  {
>  	struct task_struct *tsk = current;
>  	int group_dead;
>  
> +	if (atomic_dec_if_positive(&exit_concurrent_max) < 0) {
> +		pr_warn_ratelimited("exit: exit_concurrent_max (%u) close to 0 (max : %u), throttling...",
> +				    atomic_read(&exit_concurrent_max),
> +				    MAX_EXIT_CONCURRENT);
> +		wait_event(exit_wq,
> +			   atomic_dec_if_positive(&exit_concurrent_max) >= 0);
> +	}
> +
>  	/*
>  	 * We can get here from a kernel oops, sometimes with preemption off.
>  	 * Start by checking for critical errors.
> @@ -881,6 +894,9 @@ void __noreturn do_exit(long code)
>  
>  	lockdep_free_task(tsk);
>  	do_task_dead();

The function do_task_dead never returns.

> +
> +	atomic_inc(&exit_concurrent_max);
> +	wake_up(&exit_wq);
>  }
>  EXPORT_SYMBOL_GPL(do_exit);
>  
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 4f5613dac227..980ffaba1ac5 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -238,6 +238,8 @@ struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
>  		long max;
>  		tns = iter->ns;
>  		max = READ_ONCE(tns->ucount_max[type]);
> +		if (atomic_long_read(&iter->ucount[type]) > max/16)
> +			cond_resched();
>  		if (!atomic_long_inc_below(&iter->ucount[type], max))
>  			goto fail;
>  	}

Eric