linux-kernel - Re: Default governor regardless of cpuidle driver

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Date:   Thu, 29 Aug 2019 20:11:08 +0100
From:   Joao Martins <joao.m.martins@...cle.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, linux-pm@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: Default governor regardless of cpuidle driver

On 8/29/19 7:28 PM, Daniel Lezcano wrote:
> 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@...cle.com>
>>>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.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. 

/nods Correct.

> 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.
> 
Hmm, but even with separate kernels/configs for guest and host I think we would
still have the same issue.

What I was trying to convey is that even when running with a config solely for
KVM guests (that is different than baremetal) you can have today various ways of
idling. An Intel x86 kvm guest can have no idle driver (but arch-specific),
intel_idle (like baremetal config) and haltpoll. There are usecases for these
three, and makes sense to consolidate all.

Say you wanted to have a kvm specific config, you would still see the same
problem if you happen to compile intel_idle together with haltpoll
driver+governor. Creating two separate configs here, with and without haltpoll
for VMs doesn't sound effective for distros. Perhaps decreasing the rating of
haltpoll governor, but while a short term fix it wouldn't give much sensible
defaults without the one-off runtime switch.

Unless ofc I am missing something.

> 
>>>> 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=.
> 
>