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Message-ID: <53ECB9CD.9040705@linaro.org> Date: Thu, 14 Aug 2014 15:29:49 +0200 From: Daniel Lezcano <daniel.lezcano@...aro.org> To: Peter Zijlstra <peterz@...radead.org> CC: Chuansheng Liu <chuansheng.liu@...el.com>, "Rafael J. Wysocki" <rjw@...ysocki.net>, "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, changcheng.liu@...el.com, xiaoming.wang@...el.com, souvik.k.chakravarty@...el.com Subject: Re: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM_QOS back to DEFAULT On 08/14/2014 02:41 PM, Peter Zijlstra wrote: > On Thu, Aug 14, 2014 at 01:14:49PM +0200, Daniel Lezcano wrote: >> On 08/14/2014 01:00 PM, Peter Zijlstra wrote: >>> On Thu, Aug 14, 2014 at 12:29:32PM +0200, Daniel Lezcano wrote: >>>> Hi Chuansheng, >>>> >>>> On 14 August 2014 04:11, Chuansheng Liu <chuansheng.liu@...el.com> wrote: >>>> >>>>> We found sometimes even after we let PM_QOS back to DEFAULT, >>>>> the CPU still stuck at C0 for 2-3s, don't do the new suitable C-state >>>>> selection immediately after received the IPI interrupt. >>>>> >>>>> The code model is simply like below: >>>>> { >>>>> pm_qos_update_request(&pm_qos, C1 - 1); >>>>> < == Here keep all cores at C0 >>>>> ...; >>>>> pm_qos_update_request(&pm_qos, PM_QOS_DEFAULT_VALUE); >>>>> < == Here some cores still stuck at C0 for 2-3s >>>>> } >>>>> >>>>> The reason is when pm_qos come back to DEFAULT, there is IPI interrupt to >>>>> wake up the core, but when core is in poll idle state, the IPI interrupt >>>>> can not break the polling loop. >>> >>> So seeing how you're from @intel.com I'm assuming you're using x86 here. >>> >>> I'm not seeing how this can be possible, MWAIT is interrupted by IPIs >>> just fine, which means we'll fall out of the cpuidle_enter(), which >>> means we'll cpuidle_reflect(), and then leave cpuidle_idle_call(). >>> >>> It will indeed not leave the cpu_idle_loop() function and go right back >>> into cpuidle_idle_call(), but that will then call cpuidle_select() which >>> should pick a new C state. >>> >>> So the interrupt _should_ work. If it doesn't you need to explain why. >> >> I think the issue is related to the poll_idle state, in >> drivers/cpuidle/driver.c. This state is x86 specific and inserted in the >> cpuidle table as the state 0 (POLL). There is no mwait for this state. It is >> a bit confusing because this state is not listed in the acpi / intel idle >> driver but inserted implicitly at the beginning of the idle table by the >> cpuidle framework when the driver is registered. >> >> static int poll_idle(struct cpuidle_device *dev, >> struct cpuidle_driver *drv, int index) >> { >> local_irq_enable(); >> if (!current_set_polling_and_test()) { >> while (!need_resched()) >> cpu_relax(); >> } >> current_clr_polling(); >> >> return index; >> } > > Ah, well, in that case there's a ton more broken than just this. > kick_all_cpus_sync() won't work either, and cpuidle_reflect() pretty > much expects to be called after each interrupt. Agree. > Then again, not reflecting properly isn't really a problem, its not like > not accounting interrupts is going to safe power much. I think the main issue here is to exit the poll_idle loop when an IPI is received. IIUC, there is a pm_qos user, perhaps a driver (Chuansheng can give more details), setting a very short latency, so the cpuidle framework choose a shallow state like the poll_idle and then the driver sets a bigger latency, leading to the IPI to wake all the cpus. As the CPUs are in the poll_idle, they don't exit until an event make them to exit the need_resched() loop (reschedule or whatever). This situation can let the CPUs to stand in the infinite loop several seconds while we are expecting them to exit the poll_idle and enter a deeper idle state, thus with an extra energy consumption. -- <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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