linux-kernel - Re: [PATCH] printk: avoid livelock if another CPU printks continuously

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Date:	Wed, 10 Feb 2016 17:10:16 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	linux-kernel@...r.kernel.org, srostedt@...hat.com,
	Steven Rostedt <rostedt@...dmis.org>,
	Tejun Heo <tj@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>,
	Jan Kara <jack@...e.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kyle McMartin <kyle@...nel.org>,
	KY Srinivasan <kys@...rosoft.com>,
	Dave Jones <davej@...emonkey.org.uk>,
	Calvin Owens <calvinowens@...com>
Subject: Re: [PATCH] printk: avoid livelock if another CPU printks
 continuously

On Wed 2016-02-10 09:44:07, Steven Rostedt wrote:
> On Wed, 10 Feb 2016 15:36:49 +0100
> Petr Mladek <pmladek@...e.com> wrote:
> 
> > Bcc: 
> > Subject: Re: [PATCH] printk: avoid livelock if another CPU printks
> >  continuously
> > Reply-To: 
> > In-Reply-To: <1454963703-20433-1-git-send-email-dvlasenk@...hat.com>
> > 
> 
> Hmm, playing with mail headers?

Yeah. I am trying to do this reply back in the original thread.

> > > +				/* Good, other CPU entered "for(;;)" loop */
> > > +				goto out;
> > > +			}
> > > +		}
> > > +		/* No one seems to be willing to take it... */
> > > +		if (console_trylock())
> > > +			goto again; /* we took it */
> > > +		/* Nope, someone else holds console_sem! Good */  
> > 
> > The cycle gives a big chance other CPUs to enter console_unlock().
> > It means that more CPUs might end up in the above busy cycle.
> > 
> > It gives a chance to move the printing to another CPU. It likely
> > slows down the flood of messages because the producer end up
> > here as well.
> > 
> > So, it probably works but the performance is far from optimal.
> > Many CPUs might end up doing nothing. I am afraid that this is
> > not the right way to go.
> 
> Note, it's not that performance critical, and the loop only happens if
> someone else is adding to the console, which hopefully, should be rare.

I probably used too strong words. It is possible that the performance
impact will not be critical. But the behavior is non-deterministic.
I think that the approach taken by Jack is more promising.
I mean the offloading of the console stuff to a workqueue.

Best Regards,
Petr