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From: Nicolai Stange <nicstange@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Nicolai Stange <nicstange@gmail.com>,
        John Stultz <john.stultz@linaro.org>, linux-kernel@vger.kernel.org
Subject: Re: [RFC v7 00/23] adapt clockevents frequencies to mono clock
References: <20160916200851.9273-1-nicstange@gmail.com>
        <alpine.DEB.2.20.1609202251290.5476@nanos> <877fa641aj.fsf@gmail.com>
        <alpine.DEB.2.20.1609210133510.5476@nanos>
Date: Wed, 21 Sep 2016 16:06:43 +0200
In-Reply-To: <alpine.DEB.2.20.1609210133510.5476@nanos> (Thomas Gleixner's
        message of "Wed, 21 Sep 2016 01:36:22 +0200 (CEST)")
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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.

Unfortunately, the invocation frequency doesn't calm down as much as it
did on x86_64: after an uptime of 45min, I'm still seeing approximately
one invocation per second. Right after boot, it was ~3/s.

Thanks,

Nicolai