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

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Date:	Fri, 5 Mar 2010 19:43:59 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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, Mar 05, 2010 at 01:35:00PM -0500, Steven Rostedt wrote:
> 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!



Ouch...that's the hardirqs_off_events/redundant_hardirqs_off variables?
Those should be clearly made per cpu.

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