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
