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Message-ID: <545B6932.4010308@linaro.org> Date: Thu, 06 Nov 2014 13:27:30 +0100 From: Daniel Lezcano <daniel.lezcano@...aro.org> To: Preeti U Murthy <preeti@...ux.vnet.ibm.com> CC: "Rafael J. Wysocki" <rjw@...ysocki.net>, Nicolas Pitre <nicolas.pitre@...aro.org>, "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Lists linaro-kernel <linaro-kernel@...ts.linaro.org>, patches@...aro.org Subject: Re: [PATCH V2 1/5] sched: idle: cpuidle: Check the latency req before idle On 11/06/2014 05:08 AM, Preeti U Murthy wrote: > 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 >>>>> <daniel.lezcano@...aro.org> 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. I am not sure. Could you describe what is the difference between the arch_cpu_idle function in arch/arm/powerpc/kernel/idle.c and the 0th power PC idle state ? Is it kind of duplicate ? And for polling, do you really want to use while (...); cpu_relax(); as it is x86 specific ? instead of the powerpc's arch_idle ? Today, if latency_req == 0, it returns the 0th idle state, so polling. If we jump to the arch_cpu_idle_poll, the result will be the same for all architecture. >> 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? It sounds a bit hackish. I prefer to sort out the current situation. And by the way, what is the reasoning behind having a target_residency / exit_latency equal to zero for an idle state ? All this sounds really fuzzy for me. >> 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. >> >> >> >> >> > -- <http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | <http://twitter.com/#!/linaroorg> Twitter | <http://www.linaro.org/linaro-blog/> Blog -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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