linux-kernel - Re: [RFC v7 00/23] adapt clockevents frequencies to mono clock

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Date:   Wed, 21 Sep 2016 01:36:22 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Nicolai Stange <nicstange@...il.com>
cc:     John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC v7 00/23] adapt clockevents frequencies to mono clock

On Wed, 21 Sep 2016, Nicolai Stange wrote:
> Thomas Gleixner <tglx@...utronix.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.

Thanks,

	tglx