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 <[email protected]> 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. Then again, not reflecting properly isn't really a problem, its not like not accounting interrupts is going to safe power much.
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