From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758223AbcIVWjq (ORCPT ); Thu, 22 Sep 2016 18:39:46 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:36626 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758200AbcIVWjo (ORCPT ); Thu, 22 Sep 2016 18:39:44 -0400 From: Nicolai Stange To: Thomas Gleixner Cc: Nicolai Stange , John Stultz , linux-kernel@vger.kernel.org Subject: Re: [RFC v7 00/23] adapt clockevents frequencies to mono clock References: <20160916200851.9273-1-nicstange@gmail.com> <877fa641aj.fsf@gmail.com> <874m59cpos.fsf@gmail.com> Date: Fri, 23 Sep 2016 00:39:41 +0200 In-Reply-To: (Thomas Gleixner's message of "Thu, 22 Sep 2016 23:39:00 +0200 (CEST)") Message-ID: <87shsrlfte.fsf@gmail.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Thomas Gleixner writes: > On Wed, 21 Sep 2016, Nicolai Stange wrote: >> Thomas Gleixner writes: >> >> > On Wed, 21 Sep 2016, Nicolai Stange wrote: >> >> Thomas Gleixner 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? I think this is because the histogram has got two peaks, c.f. [1] Also, the 11us maximum is not isolated but a flat tail is reaching to this point which I admittedly can't explain. > 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. AFACIT, the "algorithmic" runtime should be constant per CED, so it should not be dependent on any input parameters. > 11 us on that ARM worries me. I'll try to do some more tracing tomorrow in order to get the reason for that histogram's long tail. But I have to admit that I don't really know what to look for except for NMIs. Any hints? What might be remarkable in this context is that the dataset's min is at 2.24us. Perhaps I'm actually seeing the distribution of the clockevents_lock acquisition? Thanks, Nicolai [1] https://nicst.de/cev-freqadjust/adjust_all_freqs-function_graph_hist.png