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Date: Fri, 17 Apr 2009 17:24:13 -0600
From: "Chris Friesen" <cfriesen@nortel.com>
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To: Christoph Lameter <cl@linux.com>
CC: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
       linux-kernel@vger.kernel.org
Subject: Re: Scheduler regression: Too frequent timer interrupts(?)
References: <alpine.DEB.1.10.0904161539130.23632@qirst.com>  <1239951613.23397.4107.camel@laptop>  <alpine.DEB.1.10.0904170941020.11877@qirst.com>  <1239977776.23397.4590.camel@laptop>  <alpine.DEB.1.10.0904171021360.20239@qirst.com>  <1239979901.23397.4638.camel@laptop>  <alpine.DEB.1.10.0904171100510.21575@qirst.com>  <20090417153520.GA29968@elte.hu>  <alpine.DEB.1.10.0904171142050.13734@qirst.com>  <1239985426.23397.4757.camel@laptop> <20090417164918.GK8253@elte.hu>  <alpine.DEB.1.10.0904171335490.28603@qirst.com> <1239991892.23397.4905.camel@laptop> <alpine.DEB.1.10.0904171410540.28536@qirst.com>
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Christoph Lameter wrote:

> The latencytest code simulates a busy processor (no system calls, all
> memory is prefaulted). For some reasons Linux is increasingly taking time
> away from such processes (that intentionally run uncontended on a
> dedicated processor). This causes regressions so that current upstream is
> not usable for these applications.
> 
> It would be best for these applications if the processor would be left
> undisturbed. There is likely not much that the OS needs to do on a busy
> processor if there are no competing threads and if there is no I/O taking
> place.

Peter/Ingo, could ftrace be used to determine where time is being spent 
in the kernel with suitable accuracy?   (This may be a dumb question, I 
haven't played with ftrace much.)

Given that we're talking about tens of usecs of duration, statistical 
sampling may not work all that well.

Chris