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

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Message-ID: <877fa641aj.fsf@gmail.com>
Date:   Wed, 21 Sep 2016 01:08:36 +0200
From:   Nicolai Stange <nicstange@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Nicolai Stange <nicstange@...il.com>,
        John Stultz <john.stultz@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v7 00/23] adapt clockevents frequencies to mono clock

Thomas Gleixner <tglx@...utronix.de> writes:

> On Fri, 16 Sep 2016, Nicolai Stange wrote:
>
>> Goal: avoid programming ced devices too early for large deltas, for
>>       details, c.f. the description of [21/23].
>> 
>> [21-23/23] Actually do the frequency adjustments.
>> 
>> Tested on x86_64 and next-20160916.
>
> 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):

# tracer: function_graph
#
#     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();
  149.507660 |   2)   2.308 us    |  clockevents_adjust_all_freqs();
  149.511658 |   2)   2.651 us    |  clockevents_adjust_all_freqs();
  149.545660 |   0)   2.268 us    |  clockevents_adjust_all_freqs();
  149.564211 |   2)   2.321 us    |  clockevents_adjust_all_freqs();
  214.351899 |   2)   1.520 us    |  clockevents_adjust_all_freqs();
  214.354935 |   0)   1.053 us    |  clockevents_adjust_all_freqs();
  279.026205 |   0)   2.289 us    |  clockevents_adjust_all_freqs();
  279.030195 |   0)   2.190 us    |  clockevents_adjust_all_freqs();
  279.034196 |   0)   2.381 us    |  clockevents_adjust_all_freqs();
  279.047492 |   2)   2.390 us    |  clockevents_adjust_all_freqs();
  344.250356 |   1)   2.727 us    |  clockevents_adjust_all_freqs();
  408.879538 |   1)   2.235 us    |  clockevents_adjust_all_freqs();
  473.125730 |   6)   1.513 us    |  clockevents_adjust_all_freqs();
  473.129731 |   6)   1.650 us    |  clockevents_adjust_all_freqs();
  538.387891 |   3)   2.305 us    |  clockevents_adjust_all_freqs();
  538.391890 |   3)   2.300 us    |  clockevents_adjust_all_freqs();
  668.257162 |   3)   2.691 us    |  clockevents_adjust_all_freqs();
  668.261162 |   3)   2.306 us    |  clockevents_adjust_all_freqs();
  733.459261 |   0)   1.066 us    |  clockevents_adjust_all_freqs();
  733.463261 |   0)   1.233 us    |  clockevents_adjust_all_freqs();
  733.467263 |   1)   1.382 us    |  clockevents_adjust_all_freqs();
  863.398561 |   2)   2.218 us    |  clockevents_adjust_all_freqs();
  863.402552 |   2)   2.792 us    |  clockevents_adjust_all_freqs();
 1122.210001 |   3)   2.259 us    |  clockevents_adjust_all_freqs();
 1122.214004 |   3)   2.165 us    |  clockevents_adjust_all_freqs();
 1381.283287 |   2)   1.944 us    |  clockevents_adjust_all_freqs();
 1895.664008 |   2)   1.940 us    |  clockevents_adjust_all_freqs();
 1895.668009 |   2)   2.041 us    |  clockevents_adjust_all_freqs();
 2930.385388 |   0)   1.067 us    |  clockevents_adjust_all_freqs();
 2930.386390 |   5)   1.208 us    |  clockevents_adjust_all_freqs();


Thanks,

Nicolai