linux-kernel - Re: Reading EeePC900 battery info causes stalls

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Date:	Sat, 20 Sep 2008 17:53:20 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sitsofe Wheeler <sitsofe@...oo.com>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, lenb@...nel.org,
	astarikovskiy@...e.de
Subject: Re: Reading EeePC900 battery info causes stalls


On Sat, 20 Sep 2008, Sitsofe Wheeler wrote:

> Steven Rostedt wrote:
> > OK, that is probably the known bug you are hitting. Simply disable the
> > CONFIG_FTRACE_STARTUP_TEST and you should have the wakeup tracer. The bug is
> > with the test, not the tracer, so it should not hurt you.
> 
> Thanks - this made the wakeup tracer appear a you said. I have put two wakeup
> traces up:
> 
> http://sucs.org/~sits/test/eeepc-debug/20080920/latency_trace.txt.gz
> http://sucs.org/~sits/test/eeepc-debug/20080920/trace.txt.gz
> (each file is around 6Mbytes uncompressed)
> 
> Here's a small extract of latency_trace.txt:
> 
> > # tracer: wakeup
> > #
> > wakeup latency trace v1.1.5 on 2.6.27-rc6skw-dirty
> > --------------------------------------------------------------------
> >  latency: 3232905 us, #65620/6180619, CPU#0 | (M:desktop VP:0, KP:0, SP:0

Peter, these times are crazy, mainly due to the cpu_clock. He probably 
wants to use the sched_clock. Below is a patch to use it instead.

Sitsofe, I notice that the trace states "desktop". This means that you
are running with CONFIG_PREEMPT_VOLUNTARY. You want 
CONFIG_PREEMPT.

[...]
> 
> Is it intentional that the last event has a time earlier closer to that of the
> first event?
> 

Change the config, and see what you get with this patch:

Note this is not compiled tested:

---
 kernel/trace/trace.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linus.git/kernel/trace/trace.c
===================================================================
--- linus.git.orig/kernel/trace/trace.c	2008-09-20 17:49:00.000000000 -0400
+++ linus.git/kernel/trace/trace.c	2008-09-20 17:49:35.000000000 -0400
@@ -60,7 +60,7 @@ ns2usecs(cycle_t nsec)
 
 cycle_t ftrace_now(int cpu)
 {
-	return cpu_clock(cpu);
+	return sched_clock();
 }
 
 /*


-- Steve

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