Re: How to avoid printk() delay caused by cond_resched() ?

[Petr Mladek <pmladek@suse.com>]

On Wed 2016-03-02 23:11:30, Tetsuo Handa wrote:
> That's a good news. I was wishing that there were a dedicated kernel
> thread which does printk() operation. While at it, I ask for an API
> which waits for printk buffer to be flushed (something like below) so that
> a watchdog thread which might dump thousands of threads from sleepable
> context (like my dump) can avoid "** XXX printk messages dropped **"
> messages.
>
> ----------
> diff --git a/include/linux/console.h b/include/linux/console.h
> index ea731af..11e936c 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -147,6 +147,7 @@ extern int unregister_console(struct console *);
>  extern struct console *console_drivers;
>  extern void console_lock(void);
>  extern int console_trylock(void);
> +extern void wait_console_flushed(unsigned long timeout);
>  extern void console_unlock(void);
>  extern void console_conditional_schedule(void);
>  extern void console_unblank(void);
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 9917f69..2eb60df 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -121,6 +121,15 @@ static int __down_trylock_console_sem(unsigned long ip)
>  	up(&console_sem);\
>  } while (0)
>  
> +static int __down_timeout_console_sem(unsigned long timeout, unsigned long ip)
> +{
> +	if (down_timeout(&console_sem, timeout))
> +		return 1;
> +	mutex_acquire(&console_lock_dep_map, 0, 1, ip);
> +	return 0;
> +}
> +#define down_timeout_console_sem(timeout) __down_timeout_console_sem((timeout), _RET_IP_)
> +
>  /*
>   * This is used for debugging the mess that is the VT code by
>   * keeping track if we have the console semaphore held. It's
> @@ -2125,6 +2134,21 @@ int console_trylock(void)
>  }
>  EXPORT_SYMBOL(console_trylock);
>  
> +void wait_console_flushed(unsigned long timeout)
> +{
> +	might_sleep();
> +
> +	if (down_timeout_console_sem(timeout))
> +		return;
> +	if (console_suspended) {
> +		up_console_sem();
> +		return;
> +	}
> +	console_locked = 1;
> +	console_may_schedule = 1;
> +	console_unlock();
> +}

This tries to take over the responsibility for printing to the
console. I would personally solve this by a wait queue.
console_unlock() might wakeup all waiters when empty. This
will work also when the console stuff is offloaded into
the workqueue.

But there still might be dropped messages if there is a flood
of them from another process. Note that even userspace could push
messages into the kernel ring buffer via /dev/kmsg. We need
to be careful against DOS attacks.

Best Regards,
Petr

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