linux-kernel - Re: [rc8] Regression: key repeats and skips in music during compilation

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Date:	Tue, 8 Apr 2008 15:54:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frans Pop <elendil@...net.nl>
Cc:	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [rc8] Regression: key repeats and skips in music during
	compilation


* Frans Pop <elendil@...net.nl> wrote:

> > and a more laborous way would be to trace such skipping:
> >  http://people.redhat.com/mingo/sched-devel.git/howto-trace-latencies.txt
> 
> Tried this too, but the problem is that the skips don't happen while 
> tracing is enabled. Apparently the system overhead is so high that it 
> changes the scheduling behavior. Attached one trace that was made 
> immediately after a skip happened. Maybe it helps.

one way to reduce tracing overhead would be:

  echo nostacktrace > /sys/kernel/debug/tracing/iter_ctrl
  echo nosched-tree > /sys/kernel/debug/tracing/iter_ctrl

and making sure you only have CONFIG_CONTEXT_SWITCH_TRACER=y enabled, 
not any of the other tracers.

but ... if there's a narrow wakeup race, even lightweight tracing could 
make it go away.

	Ingo


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