Re: [PATCH] mm: don't warn about allocations which stall for too long

[Petr Mladek]

On Wed 2017-11-01 09:30:05, Vlastimil Babka wrote:
> On 10/31/2017 08:32 PM, Steven Rostedt wrote:
> > 
> > Thank you for the perfect timing. You posted this the day after I
> > proposed a new solution at Kernel Summit in Prague for the printk lock
> > loop that you experienced here.
> > 
> > I attached the pdf that I used for that discussion (ignore the last
> > slide, it was left over and I never went there).
> > 
> > My proposal is to do something like this with printk:
> > 
> > Three types of printk usages:
> > 
> > 1) Active printer (actively writing to the console).
> > 2) Waiter (active printer, first user)
> > 3) Sees active printer and a waiter, and just adds to the log buffer
> >    and leaves.
> > 
> > (new globals)
> > static DEFINE_SPIN_LOCK(console_owner_lock);
> > static struct task_struct console_owner;
> > static bool waiter;
> > 
> > console_unlock() {
> > 
> > [ Assumes this part can not preempt ]
> > 
> > 	spin_lock(console_owner_lock);
> > 	console_owner = current;
> > 	spin_unlock(console_owner_lock);
> > 
> > 	for each message
> > 		write message out to console
> > 
> > 		if (READ_ONCE(waiter))
> > 			break;
> 
> Ah, these two lines clarified for me what I didn't get from your talk,
> so I got the wrong impression that the new scheme is just postponing the
> problem.
> 
> But still, it seems to me that the scheme only works as long as there
> are printk()'s coming with some reasonable frequency. There's still a
> corner case when a storm of printk()'s can come that will fill the ring
> buffers, and while during the storm the printing will be distributed
> between CPUs nicely, the last unfortunate CPU after the storm subsides
> will be left with a large accumulated buffer to print, and there will be
> no waiters to take over if there are no more printk()'s coming. What
> then, should it detect such situation and defer the flushing?

This was my fear as well. Steven argued that this was theoretical.
And I do not have a real-life bullets against this argument at
the moment.

My current main worry with Steven's approach is a risk of deadlocks
that Jan Kara saw when he played with similar solution.

Also I am afraid that it would add yet another twist to the console
locking operations. It is already quite hard to follow the logic,
see the games with:

	+ console_locked
	+ console_suspended
	+ can_use_console()
	+ exclusive_console

And Steven is going to add:

	+ console_owner
	+ waiter

But let's wait for the patch. It might look and work nicely
in the end.

Best Regards,
Petr