Re: [PATCH V3 2/6] sched: idle: cpuidle: Check the latency req before idle

Daniel Lezcano
2014-11-10T15:58:35Z

On 11/10/2014 04:28 PM, Peter Zijlstra wrote:
> On Mon, Nov 10, 2014 at 04:12:47PM +0100, Daniel Lezcano wrote:
>> All this is to remove the poll idle state from the x86 cpuidle driver in
>> order to remove the CPUIDLE_DRIVER_STATE_START (this one forces to write
>> always ugly code in the cpuidle framework).
>>
>> This poll state introduces the CPUIDLE_DRIVER_STATE_START macro. If you look
>> at the different governors and the code, you can checkout what kind of
>> tricks this macro introduces and how that makes the code ugly.
>>
>> For the sake of what ? Just a small optimization in the menu governor.
>>
>> I suppose that has been introduce and then evolved on a wrong basis. So now
>> we have to deal with that.
>>
>> This patchset provides a first round of cleanup around the poll function,
>> the next patchset will move the 5us timer optimization from the menu
>> governor and the last patchset will remove the CPUIDLE_DRIVER_STATE_START
>> ugly macro.
>
> I don't get it, I've clearly not stared at it long enough, but why is
> that STATE_START crap needed anywhere?

Excellent question :)

On x86, the config option ARCH_HAS_CPU_RELAX is set (x86 is the only
one). That leads to the macro CPUIDLE_DRIVER_STATE_START equal 1.

https://git.kernel.org/cgit/linux/kernel/git/rafael/linux-pm.git/tree/include/linux/cpuidle.h#n221

Then the acpi cpuidle driver and the intel_driver begin to fill the idle
state at index == CPUIDLE_DRIVER_STATE_START, so leaving the 0th idle
state empty.

https://git.kernel.org/cgit/linux/kernel/git/rafael/linux-pm.git/tree/drivers/idle/intel_idle.c#n848

https://git.kernel.org/cgit/linux/kernel/git/rafael/linux-pm.git/tree/drivers/acpi/processor_idle.c#n953

Then when the driver is registered and if ARCH_HAS_CPU_RELAX is set, the
cpuidle framework insert the 0th with the poll state (reminder : only
for x86).

https://git.kernel.org/cgit/linux/kernel/git/rafael/linux-pm.git/tree/drivers/cpuidle/driver.c#n195

If you look at the ladder governor (which I believe nobody is still
using it), or at the menu governor, all the indexes begin at
CPUIDLE_DRIVER_STATE_START, so all the code is preventing to use the 0th
state ... :)

So why is needed the poll state ?

1. When the latency_req is 0 (it returns 0, so the poll state)

2. When the select's menu governor fails to find a state *and* if the
next timer is before 5us

https://git.kernel.org/cgit/linux/kernel/git/rafael/linux-pm.git/tree/drivers/cpuidle/driver.c#n195

And when we investigate the same code but on the other archs, the
CPUIDLE_DRIVER_STATE_START dance makes things slightly different.

So the conclusion is, we are inserting a state in the idle state array
but we do everything to prevent to use it :)

For me it appears logical to just kill this state from the x86 idle
drivers and move it in the idle_mainloop in case an idle state selection
fails.

> To me it appears 'natural' to have a latency_req==0 state, why does it
> need so much special casing?

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