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Date: Wed, 10 Feb 2016 17:10:16 +0100
From: Petr Mladek <pmladek@suse.com>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: linux-kernel@vger.kernel.org, srostedt@redhat.com,
        Steven Rostedt <rostedt@goodmis.org>, Tejun Heo <tj@kernel.org>,
        Peter Hurley <peter@hurleysoftware.com>, Jan Kara <jack@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kyle McMartin <kyle@kernel.org>, KY Srinivasan <kys@microsoft.com>,
        Dave Jones <davej@codemonkey.org.uk>,
        Calvin Owens <calvinowens@fb.com>
Subject: Re: [PATCH] printk: avoid livelock if another CPU printks
 continuously
Message-ID: <20160210161016.GF12548@pathway.suse.cz>
References: <1454963703-20433-1-git-send-email-dvlasenk@redhat.com>
 <20160210144416.GE12548@pathway.suse.cz>
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On Wed 2016-02-10 09:44:07, Steven Rostedt wrote:
> On Wed, 10 Feb 2016 15:36:49 +0100
> Petr Mladek <pmladek@suse.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@redhat.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