From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mason Subject: Re: Plain DFS (no voltage scaling) Date: Wed, 3 Feb 2016 19:19:12 +0100 Message-ID: <56B244A0.60309@free.fr> References: <56B11B67.3090600@free.fr> <56B217C9.1050405@free.fr> <20160203161331.GI3469@vireshk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Return-path: Received: from smtp4-g21.free.fr ([212.27.42.4]:20965 "EHLO smtp4-g21.free.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965637AbcBCSTV (ORCPT ); Wed, 3 Feb 2016 13:19:21 -0500 In-Reply-To: <20160203161331.GI3469@vireshk> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Viresh Kumar Cc: Linux PM , "Rafael J. Wysocki" , Linux ARM On 03/02/2016 17:13, Viresh Kumar wrote: > On 03-02-16, 16:07, Mason wrote: >> On 02/02/2016 22:11, Mason wrote: >> >>> I plan to enable the on-demand governor on the tango platform: >>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/tango4-smp8758.dtsi >>> >>> I found the cpufreq-dt binding doc: >>> >>> https://www.kernel.org/doc/Documentation/devicetree/bindings/cpufreq/cpufreq-dt.txt >>> https://www.kernel.org/doc/Documentation/devicetree/bindings/opp/opp.txt >>> >>> Something is not clear to me: >>> >>> If my platform cannot scale the voltage, what information >>> should I put in the voltage part of the DT? >> >> Someone pointed out that tweaking the frequency without tweaking >> the voltage might be counter-productive. >> >> I measured the power consumption of the entire board (at the power outlet) >> for 3 CPU frequencies (all other things being equal, I hope). >> >> idle @ 111 MHz = 4.6 W >> idle @ 333 MHz = 4.6 W >> idle @ 999 MHz = 4.6 W >> >> load @ 111 MHz = 5.0 W >> load @ 333 MHz = 5.7 W >> load @ 999 MHz = 7.7 W >> >> When idle, the kernel calls WFI, which "turns off" most of the CPU >> (clock gating?) such that the actual frequency does not matter. >> >> At full load (I use cpuburn to jog as many FUs simultaneously as >> possible) it looks like each additional MHz requires ~3 mW. >> >> So it would appear that an on-demand governor might not help to >> save power. > > Why do you say so ? Here's my (possibly incorrect) reasoning: If the CPU is idle, kernel calls WFI and frequency apparently doesn't matter. If the CPU has work to do, the on-demand governor will bump the frequency to the max (I think). I don't think the CPU spends a lot of time in the intermediate frequencies (neither max nor min). But maybe my logic is flawed? Also, some users (of our older kernel) reported problems were they considered the on-demand governor was "too slow" to ramp the frequency up. (But I don't know if they'd played with the configuration knobs.) Regards. From mboxrd@z Thu Jan 1 00:00:00 1970 From: slash.tmp@free.fr (Mason) Date: Wed, 3 Feb 2016 19:19:12 +0100 Subject: Plain DFS (no voltage scaling) In-Reply-To: <20160203161331.GI3469@vireshk> References: <56B11B67.3090600@free.fr> <56B217C9.1050405@free.fr> <20160203161331.GI3469@vireshk> Message-ID: <56B244A0.60309@free.fr> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 03/02/2016 17:13, Viresh Kumar wrote: > On 03-02-16, 16:07, Mason wrote: >> On 02/02/2016 22:11, Mason wrote: >> >>> I plan to enable the on-demand governor on the tango platform: >>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/tango4-smp8758.dtsi >>> >>> I found the cpufreq-dt binding doc: >>> >>> https://www.kernel.org/doc/Documentation/devicetree/bindings/cpufreq/cpufreq-dt.txt >>> https://www.kernel.org/doc/Documentation/devicetree/bindings/opp/opp.txt >>> >>> Something is not clear to me: >>> >>> If my platform cannot scale the voltage, what information >>> should I put in the voltage part of the DT? >> >> Someone pointed out that tweaking the frequency without tweaking >> the voltage might be counter-productive. >> >> I measured the power consumption of the entire board (at the power outlet) >> for 3 CPU frequencies (all other things being equal, I hope). >> >> idle @ 111 MHz = 4.6 W >> idle @ 333 MHz = 4.6 W >> idle @ 999 MHz = 4.6 W >> >> load @ 111 MHz = 5.0 W >> load @ 333 MHz = 5.7 W >> load @ 999 MHz = 7.7 W >> >> When idle, the kernel calls WFI, which "turns off" most of the CPU >> (clock gating?) such that the actual frequency does not matter. >> >> At full load (I use cpuburn to jog as many FUs simultaneously as >> possible) it looks like each additional MHz requires ~3 mW. >> >> So it would appear that an on-demand governor might not help to >> save power. > > Why do you say so ? Here's my (possibly incorrect) reasoning: If the CPU is idle, kernel calls WFI and frequency apparently doesn't matter. If the CPU has work to do, the on-demand governor will bump the frequency to the max (I think). I don't think the CPU spends a lot of time in the intermediate frequencies (neither max nor min). But maybe my logic is flawed? Also, some users (of our older kernel) reported problems were they considered the on-demand governor was "too slow" to ramp the frequency up. (But I don't know if they'd played with the configuration knobs.) Regards.