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

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Message-ID: <20071025201948.GO17536@waste.org>
Date:	Thu, 25 Oct 2007 15:19:48 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Tim Bird <tim.bird@...sony.com>
Cc:	linux kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: IRQ off latency of printk is very high

On Thu, Oct 25, 2007 at 12:44:31PM -0700, Tim Bird wrote:
> I've been looking at 'IRQ off' latency in the Linux kernel, on
> version 2.6.22 for target using an ARM processor.
> I use a serial console, at 115200 bps.

Printk to the serial console uses polled I/O to get deterministic,
reliable, and -timely- output. If our very next statement (or interrupt)
may lock up the box, we want to be sure our printk has actually been
delivered before that happens.

Kindof a bummer for realtime, but also rather hard to get around.

> I've noticed that calls to printk disable interrupts for
> excessively long times.  I have a long test printk of
> over 200 chars, that holds interrupts off for 24 milliseconds.

2000bits @ 115200bps -> 17.4ms

> Are these are really needed, with all this other locking
> going on?  Any ideas for fixing this?

Well, we could have a commandline option that made messages with a
priority below X go out buffered. But it'd be a lousy default from a
debugging perspective.

-- 
Mathematics is the supreme nostalgia of our time.
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