Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

[Sergey Senozhatsky]

Hi,

On (01/15/18 12:50), Petr Mladek wrote:
> On Mon 2018-01-15 11:17:43, Petr Mladek wrote:
> > PS: Sergey, you have many good points. The printk-stuff is very
> > complex and we could spend years discussing the perfect solution.
> 
> BTW: One solution that comes to my mind is based on ideas
> already mentioned in this thread:
> 
> void console_unlock(void)
> {
> 	disable_preemtion();
> 
> 	while(pending_message) {
> 
> 	    call_console_drivers();
> 
> 	    if (too_long_here() && current != printk_kthread) {
> 	       wake_up_process(printk_kthread())
> 
> 	}
> 
> 	enable_preemtion();
> }

unfortunately disabling preemtion in console_unlock() is a bit
dangerous :( we have paths that call console_unlock() exactly
to flush everything (not only new pending messages, but everything)
that is in logbuf and we cannot return from console_unlock()
preliminary in that case.

> bool too_long_here(void)
> {
> 	return should_resched();
> or
> 	return spent_here() > 1 / HZ / 2;
> or
> 	what ever we agree on
> }
> 
> 
> int printk_kthread_func(void *data)
> {
> 	while(1) {
> 		 if (!pending_messaged)
> 			schedule();
> 
> 		if (console_trylock_spinning())
> 			console_unlock();
> 
> 		cond_resched();
> 	}
> }

overall that's very close to what I have in one of my private branches.
console_trylock_spinning() for some reason does not perform really
well on my made-up internal printk torture tests. it seems that I
have a much better stability (no lockups and so on) when I also let
printk_kthread to sleep on console_sem(). but I will look further.

	-ss