linux-kernel - Re: Latency issues with x86.git

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Date:	Thu, 14 Feb 2008 07:07:58 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Kevin Winchester <kjwinchester@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Latency issues with x86.git

* Kevin Winchester <kjwinchester@...il.com> wrote:

> Hi Ingo,
> 
> I have encountered (a handful of times in the past few months) some 
> real interactivity problems on my system.  Moving the mouse or typing 
> a key on the keyboard takes around a second to show any response.  
> Once I perform a reboot, the problem is gone again.  I am currently 
> running x86.git mm branch, but I switch between that branch, mainline 
> git, and mm kernels, so I cannot guarantee on which trees I have or 
> have not seen the problem.

please try sched-devel.git, which has both the latest scheduler fixes, 
and also the new "ftrace" latency tracing framework that can be used to 
trace various latency problems. You can pick up sched-devel.git via:

   http://people.redhat.com/mingo/sched-devel.git/README

firstly, there's a chance that sched-devel.git solves the problem - in 
that case please report it.

if it doesnt, then you can trace various latencies via these:

 CONFIG_FTRACE=y
 CONFIG_IRQSOFF_TRACER=y
 CONFIG_SCHED_TRACER=y
 CONFIG_CONTEXT_SWITCH_TRACER=y
 CONFIG_DYNAMIC_FTRACE=y

just enable them, boot into the new kernel and mount debugfs:

   mount -t debugfs nodev /debug

and then you can select one of the tracers (that have been enabled in 
the .config) via /debug/tracing/* files. The usage of these files should 
be self-explaining - if any of them wasnt then please let us know and 
we'll make the "first quick glance experience" better :)

the one interesting to you would be the "wakeup" tracer. Enable it, and 
if you echo 0 into /debug/tracing/tracing_max_latency it starts tracking 
the worst-case delay experienced on your system and should start 
reporting them to the syslog.

if you encounter any problems during these steps then please let us know 
- this code is quite fresh so expect some rough edges.

	Ingo
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