Messages in this thread | | | Date | Thu, 14 Aug 2014 13:14:49 +0200 | From | Daniel Lezcano <> | Subject | Re: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM_QOS back to DEFAULT |
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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@intel.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; }
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