linux-kernel - Re: Scheduler regression: Too frequent timer interrupts(?)

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Message-Id: <1239977776.23397.4590.camel@laptop>
Date:	Fri, 17 Apr 2009 16:16:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: Scheduler regression: Too frequent timer interrupts(?)

On Fri, 2009-04-17 at 09:42 -0400, Christoph Lameter wrote:
> On Fri, 17 Apr 2009, Peter Zijlstra wrote:
> 
> > This has never been true afaikt, as long as we have a task running, we
> > take the interrupt, I just looked at the .22 code and that certainly
> > expects the scheduler_tick() to be called when there is a running
> > process.
> >
> > Also, look at /proc/interrupts if you want to determine interrupt
> > frequency.
> 
> I am mainly interested in limited the number and length of cpu holdoffs.
> 
> If the timer interrupt is always taken then it must take less than 1 usec
> in certain versions of the kernel in order to not show up in the
> statistics gathered.

Such a test as you had is pretty useless for anything. 

If you want to measure something I'd suggest making a histogram of tsc
values in 10ns buckets or something, and seeing if there are a few
predominant spikes above the noise.

With something like that you could say, the jiffy tick went from 0.8+-.1
to 1.1+-.1 us or somesuch.

After that, you could possibly use oprofile or readprofile or
perf-counters to get an idea where the time is spend. I did a quick
profile on one of my machines, and about half the kernel time spend in a
while(1) loop comes from __do_softirq().

Really, I should not have to tell you this...

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