linux-kernel - Re: IRQ off latency of printk is very high

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Message-ID: <20071101211153.GA11074@elf.ucw.cz>
Date:	Thu, 1 Nov 2007 22:11:53 +0100
From:	Pavel Machek <pavel@....cz>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Tim Bird <tim.bird@...sony.com>, Matt Mackall <mpm@...enic.com>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, Jon Smirl <jonsmirl@...oo.com>
Subject: Re: IRQ off latency of printk is very high

Hi!

> > > It seems draconian to drain the entire buffer with ints disabled.
> > > Is it possible to break this up and send out smaller chunks
> > > at a time?  Maybe by putting a chunk loop in release_console_sem()?
> > 
> > Well, I believe someone got
> > 
> > DDetetccctted ed 113223 HHzz CPUCPU
> > 
> > in his dmesg, and now we have this 'draconian' locking. How can we
> > prevent mangled messages without it?
> 
> The main interest seems to be to protect from mixed printk output
> between different CPUs in process context. I don't think it would be
> that bad if interrupts come and output error messages in the middle of a
> printk, isn't it ?
> 
> therefore, could we do something like :
> 
> 
> if (!in_irq())
>   spin_lock(&logbuf_lock);
> ...
> if (!in_irq())
>   spin_unlock(&logbuf_lock);
> 
> ? (yes, this is a crazy idea)

Two messages in atomic sections on different cpus could still be mixed
:-). But yes, something like this may be the way to go.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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