Re: Is: Default governor regardless of cpuidle driver Was: [PATCH v2] cpuidle-haltpoll: vcpu hotplug support

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 29/08/2019 20:07, Joao Martins wrote:
> On 8/29/19 6:42 PM, Daniel Lezcano wrote:
>> On 29/08/2019 19:16, Joao Martins wrote:
>>> On 8/29/19 4:10 PM, Joao Martins wrote:
>>>> When cpus != maxcpus cpuidle-haltpoll will fail to register all vcpus
>>>> past the online ones and thus fail to register the idle driver.
>>>> This is because cpuidle_add_sysfs() will return with -ENODEV as a
>>>> consequence from get_cpu_device() return no device for a non-existing
>>>> CPU.
>>>>
>>>> Instead switch to cpuidle_register_driver() and manually register each
>>>> of the present cpus through cpuhp_setup_state() callback and future
>>>> ones that get onlined. This mimmics similar logic that intel_idle does.
>>>>
>>>> Fixes: fa86ee90eb11 ("add cpuidle-haltpoll driver")
>>>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>>> ---
>>>
>>> While testing the above, I found out another issue on the haltpoll series.
>>> But I am not sure what is best suited to cpuidle framework, hence requesting
>>> some advise if below is a reasonable solution or something else is preferred.
>>>
>>> Essentially after haltpoll governor got introduced and regardless of the cpuidle
>>> driver the default governor is gonna be haltpoll for a guest (given haltpoll
>>> governor doesn't get registered for baremetal). Right now, for a KVM guest, the
>>> idle governors have these ratings:
>>>
>>>  * ladder            -> 10
>>>  * teo               -> 19
>>>  * menu              -> 20
>>>  * haltpoll          -> 21
>>>  * ladder + nohz=off -> 25
>>>
>>> When a guest is booted with MWAIT and intel_idle is probed and sucessfully
>>> registered, we will end up with a haltpoll governor being used as opposed to
>>> 'menu' (which used to be the default case). This would prevent IIUC that other
>>> C-states get used other than poll_state (state 0) and state 1.
>>>
>>> Given that haltpoll governor is largely only useful with a cpuidle-haltpoll
>>> it doesn't look reasonable to be the default? What about using haltpoll governor
>>> as default when haltpoll idle driver registers or modload.
>>
>> Are the guest and host kernel the same? IOW compiled with the same
>> kernel config?
>>
> You just need to toggle this (regardless off CONFIG_HALTPOLL_CPUIDLE):
> 
> 	CONFIG_CPU_IDLE_GOV_HALTPOLL=y
> 
> And *if you are a KVM guest* it will be the default (unless using nohz=off in
> which case ladder gets the highest rating -- see the listing right above).
> 
> Host will just behave differently because the haltpoll governor is checking if
> it is running as kvm guest, and only registering in that case.

I understood the problem. Actually my question was about if the kernels
are compiled for host and guest, and can be run indifferently. In this
case a runtime detection must be done as you propose, otherwise that can
be done at config time. I pretty sure it is the former but before
thinking about the runtime side, I wanted to double check.


>>> My idea to achieve the above would be to decrease the rating to 9 (before the
>>> lowest rated governor) and retain old defaults before haltpoll. Then we would
>>> allow a cpuidle driver to define a preferred governor to switch on idle driver
>>> registration. Naturally all of would be ignored if overidden by
>>> cpuidle.governor=.


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