linux-kernel - Re: 2.6.33: ftrace triggers soft lockup

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Date:	Thu, 4 Mar 2010 13:10:16 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	rostedt@...dmis.org
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: 2.6.33: ftrace triggers soft lockup

On Thu, Mar 4, 2010 at 11:18 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Thu, 2010-03-04 at 11:01 +0800, Américo Wang wrote:
>
>> >
>> > So it is stuck in stop machine. I wonder where exactly. I see some do_exit
>> > at the top but I wonder how much they are reliable.
>>
>> Well, I think 'kstop' is just random, sometimes I got 'watchdog' or some other
>> process.
>>
>> >
>> > Anyway, as Steve said, we really need a full config to reproduce it.
>> >
>>
>> Done in another reply.
>
> Thanks!
>
> Frederic, I notice that lockdep is on, did anything change that might
> slow down the code in lockdep, or is the function graph tracer doing
> more locking?
>
> I'm betting that we are hitting a live lock. That is, an interrupt goes
> off, it is being traced, and the function graph is tracing it, but some
> locking is happening (although it also tracks disabling of interrupts)
> and this slows the interrupt handler down enough that when it finishes,
> another interrupt goes off.
>
> Américo,
>
> Could you disable LOCKDEP and see if you still encounter this lockup?
>

Sure, after disabling LOCKDEP, I can't see the warning, _but_ the system
is still as unacceptablly slow as when LOCKDEP was enabled.

So I am afraid the problem is still there.

Thanks.
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