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

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Date:	Fri, 05 Mar 2010 13:35:00 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: 2.6.33: ftrace triggers soft lockup

On Fri, 2010-03-05 at 18:16 +0100, Frederic Weisbecker wrote:

> It's true it has a high overhead, but not to the point of
> making the whole system unusable. We are supposed to be even
> far from that. I'm currently able to turn on the function graph
> tracer and use firefox without problems. It's just a bit slower
> but it's far from a visible starvation.
> 
> And Li seems to see the same thing.
> For now I can not test, but I will try this week-end.

Americo said he's seen the issue as far back as 2.6.32. So perhaps some
CPUs take a bigger hit from the function graph tracer than others. I
have several different boxes that I can try. I've seen noticeable slow
downs but never something that cripples the box.

The only time that I've seen it cripple the box is when LOCKDEP_DEBUG
was set (which according to Americo's config it was not). But that's
because LOCKDEP_DEBUG updates a global variable every time interrupts
are enabled or disabled. This caused a huge cache line bouncing with the
function graph tracer since it caused this variable to be updated 4
times for every function call!

-- Steve

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