linux-kernel - Re: flush_to_ldisc executing 2500 times a second ?

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Message-ID: <20101220151124.GG1925@jolsa.brq.redhat.com>
Date:	Mon, 20 Dec 2010 16:11:24 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Dave Jones <davej@...hat.com>, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: flush_to_ldisc executing 2500 times a second ?

hi,

sorry for late reply, I just got back from vacation..
is this still an issue?

thanks,
jirka

On Tue, Nov 30, 2010 at 11:07:09PM -0500, Dave Jones wrote:
> While poking at an unrelated bug, I came up with a perf
> command line that showed something curious.
> 
> perf record -a -e workqueue:workqueue_execute_start sleep 1
> perf trace
> 
> shows flush_to_ldisc being executed ~2500 times.
> (http://codemonkey.org.uk/junk/flush_to_ldisc.txt)
> 
> Steve Rostedt gave me a tip about trace-cmd to get a backtrace,
> 
> trace-cmd record -e workqueue_queue_work -O stacktrace sleep 1
> 
> which yields thousands of ..
> 
>            sleep-17431 [000] 35746.452637: workqueue_queue_work: work struct=0xffff880113f2a2a0 function=flush_to_ldisc workqueue=0xffff88013f80cb68 req_cpu=0 cpu=0
>            sleep-17431 [000] 35746.452638: kernel_stack:         <stack trace>
> => __queue_work (ffffffff81068c25)
> => delayed_work_timer_fn (ffffffff81068cc8)
> => run_timer_softirq (ffffffff8105e651)
> => __do_softirq (ffffffff810571af)
> => call_softirq (ffffffff8100bbdc)
> => do_softirq (ffffffff8100d358)
> => irq_exit (ffffffff81057366)
> => smp_apic_timer_interrupt (ffffffff81482b9e)
> 
> Which doesn't really tell me much.
> 
> 
> At the time of the profiling, there's nothing being output on any tty's
> (and the profiling tools are also silent during the actual profiling) so
> I'm curious as to what's causing the flushing to be necessary.
> 
> Anyone have any ideas what's going on here ? or something further
> I could dig into ?
> 
> This is with 2.6.37-rc4.
> 
> The only other machine I have to test on right now is running 2.6.34, which
> doesn't exhibit this behaviour.  I'll try to narrow it down further tomorrow.
> 
> 	Dave
> 
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