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Date: Thu, 22 Sep 2016 23:39:00 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Nicolai Stange <nicstange@gmail.com>
cc: John Stultz <john.stultz@linaro.org>, linux-kernel@vger.kernel.org
Subject: Re: [RFC v7 00/23] adapt clockevents frequencies to mono clock
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On Wed, 21 Sep 2016, Nicolai Stange wrote:
> Thomas Gleixner <tglx@linutronix.de> writes:
> 
> > On Wed, 21 Sep 2016, Nicolai Stange wrote:
> >> Thomas Gleixner <tglx@linutronix.de> writes:
> >> > Have you ever measured the overhead of the extra work which has to be done
> >> > in clockevents_adjust_all_freqs() ?
> >> 
> >> Not exactly, I had a look at its invocation frequency which seems to
> >> decay exponentially with uptime, presumably because the NTP error
> >> approaches zero.
> >> 
> >> However, I've just gathered a function_graph ftrace on my Intel
> >> i7-4800MQ (Haswell, 8HTs):
> >> 
> >> #     TIME        CPU  DURATION                  FUNCTION CALLS
> >> #      |          |     |   |                     |   |   |   |
> >>    85.287027 |   0)   0.899 us    |  clockevents_adjust_all_freqs();
> >>    85.288026 |   0)   0.759 us    |  clockevents_adjust_all_freqs();
> >>    85.289026 |   0)   0.735 us    |  clockevents_adjust_all_freqs();
> >>    85.290026 |   0)   0.671 us    |  clockevents_adjust_all_freqs();
> >>   149.503656 |   2)   2.477 us    |  clockevents_adjust_all_freqs();
> >
> > That's not that bad. Though I'd like to see numbers for ARM (especially the
> > less powerful SoCs) as well.
> 
> On a Raspberry Pi 2B (bcm2836, ARMv7) with CONFIG_SMP=y, the mean over
> ~5300 samples is 5.14+/-1.04us with a max of 11.15us.

So why is the variance that high? You have an outlier on that intel as well
which might be caused by NMI, but it might also be a systematic issue
depending on the input parameters. 11 us on that ARM worries me.

Thanks,

	tglx