Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751313AbaKFEIf (ORCPT ); Wed, 5 Nov 2014 23:08:35 -0500 Received: from e32.co.us.ibm.com ([32.97.110.150]:52222 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751051AbaKFEIb (ORCPT ); Wed, 5 Nov 2014 23:08:31 -0500 Message-ID: <545AF424.2070302@linux.vnet.ibm.com> Date: Thu, 06 Nov 2014 09:38:04 +0530 From: Preeti U Murthy User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Daniel Lezcano CC: "Rafael J. Wysocki" , Nicolas Pitre , "linux-pm@vger.kernel.org" , LKML , Peter Zijlstra , Lists linaro-kernel , patches@linaro.org Subject: Re: [PATCH V2 1/5] sched: idle: cpuidle: Check the latency req before idle References: <1414054881-17713-1-git-send-email-daniel.lezcano@linaro.org> <544FE787.8090108@linaro.org> <54504A60.2090908@linux.vnet.ibm.com> <545A3414.7030500@linaro.org> In-Reply-To: <545A3414.7030500@linaro.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14110604-0005-0000-0000-0000062773E8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/05/2014 07:58 PM, Daniel Lezcano wrote: > On 10/29/2014 03:01 AM, Preeti U Murthy wrote: >> On 10/29/2014 12:29 AM, Daniel Lezcano wrote: >>> On 10/28/2014 04:51 AM, Preeti Murthy wrote: >>>> Hi Daniel, >>>> >>>> On Thu, Oct 23, 2014 at 2:31 PM, Daniel Lezcano >>>> wrote: >>>>> When the pmqos latency requirement is set to zero that means "poll in >>>>> all the >>>>> cases". >>>>> >>>>> That is correctly implemented on x86 but not on the other archs. >>>>> >>>>> As how is written the code, if the latency request is zero, the >>>>> governor will >>>>> return zero, so corresponding, for x86, to the poll function, but for >>>>> the >>>>> others arch the default idle function. For example, on ARM this is >>>>> wait-for- >>>>> interrupt with a latency of '1', so violating the constraint. >>>> >>>> This is not true actually. On PowerPC the idle state 0 has an >>>> exit_latency of 0. >>>> >>>>> >>>>> In order to fix that, do the latency requirement check *before* >>>>> calling the >>>>> cpuidle framework in order to jump to the poll function without >>>>> entering >>>>> cpuidle. That has several benefits: >>>> >>>> Doing so actually hurts on PowerPC. Because the idle loop defined for >>>> idle state 0 is different from what cpu_relax() does in >>>> cpu_idle_loop(). >>>> The spinning is more power efficient in the former case. Moreover we >>>> also set >>>> certain register values which indicate an idle cpu. The ppc_runlatch >>>> bits >>>> do precisely this. These register values are being read by some user >>>> space >>>> tools. So we will end up breaking them with this patch >>>> >>>> My suggestion is very well keep the latency requirement check in >>>> kernel/sched/idle.c >>>> like your doing in this patch. But before jumping to cpu_idle_loop >>>> verify if the >>>> idle state 0 has an exit_latency > 0 in addition to your check on the >>>> latency_req == 0. >>>> If not, you can fall through to the regular path of calling into the >>>> cpuidle driver. >>>> The scheduler can query the cpuidle_driver structure anyway. >>>> >>>> What do you think? >>> >>> Thanks for reviewing the patch and spotting this. >>> >>> Wouldn't make sense to create: >>> >>> void __weak_cpu_idle_poll(void) ? >>> >>> and override it with your specific poll function ? >>> >> >> No this would become ugly as far as I can see. A weak function has to be >> defined under arch/* code. We will either need to duplicate the idle >> loop that we already have in the drivers or point the weak function to >> the first idle state defined by our driver. Both of which is not >> desirable (calling into the driver from arch code is ugly). Another >> reason why I don't like the idea of a weak function is that if you have >> missed looking at a specific driver and they have an idle loop with >> features similar to on powerpc, you will have to spot it yourself and >> include the arch specific cpu_idle_poll() for them. > > Yes, I agree this is a fair point. But actually I don't see the interest > of having the poll loop in the cpuidle driver. These cleanups are We can't do that simply because the idle poll loop has arch specific bits on powerpc. > preparing the removal of the CPUIDLE_DRIVER_STATE_START macro which > leads to a lot of mess in the cpuidle code. How is the suggestion to check the exit_latency of idle state 0 when latency_req == 0 going to hinder this removal? > > With the removal of this macro, we should be able to move the select > loop from the menu governor and use it everywhere else. Furthermore, > this state which is flagged with TIME_VALID, isn't because the local > interrupt are enabled so we are measuring the interrupt time processing. > Beside that the idle loop for x86 is mostly not used. > > So the idea would be to extract those idle loop from the drivers and use > them directly when: > 1. the idle selection fails (use the poll loop under certain > circumstances we have to redefine) This behavior will not change as per my suggestion. > 2. when the latency req is zero Its only here that I suggested you also verify state 0's exit_latency. For the reason that the arch may have a more optimized idle poll loop, which we cannot override with the generic cpuidle poll loop. Regards Preeti U Murthy > > That will result in a cleaner code in cpuidle and in the governor. > > Do you agree with that ? > >> But by having a check on the exit_latency, you are claiming that since >> the driver's 0th idle state is no better than the generic idle loop in >> cases of 0 latency req, we are better off calling the latter, which >> looks reasonable. That way you don't have to bother about worsening the >> idle loop behavior on any other driver. > > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/