linux-kernel - Re: real time hang w/ latency tracing

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Date:	Wed, 04 Apr 2007 08:57:57 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: real time hang w/ latency tracing

On Wed, 2007-04-04 at 17:42 +0200, Ingo Molnar wrote:

> i mean non-printk related breakages to the normal serial driver - which 
> is now surprised by this raw lock in an -rt kernel.
> 
> this is a hard issue and there's no easy solution i can see. neither 
> having it as a raw lock is a good solution, nor having it as a 
> preemptible lock is a good solution.

The machine that I'm using is relatively fast, but on slower machine
it's possible to reproduce this same hang just by running,

echo 0 > /proc/sys/kernel/preempt_max_latency

then pressing enter a few times .. Which would be a fairly common
happening with people using latency tracing an a serial port I would
imagine..

The other fix I test was using the hack for sysrq,

        if (up->port.sysrq || oops_in_progress)

in serial8250_console_write to then also check "irqs_disabled()" , but
I'm sure mass usage of the serial port would cause some issues with
that.. Although both fixes I tested resulted in the serial port becoming
garbled after about 10 min. with the preempt_thresh at 1 .

Daniel

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