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

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Message-ID: <6c8816af-934a-5bf7-6fb9-f67c05e2c8aa@oracle.com>
Date:   Thu, 29 Aug 2019 19:07:49 +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: Is: Default governor regardless of cpuidle driver Was: [PATCH v2]
 cpuidle-haltpoll: vcpu hotplug support

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.

> 
>> 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=.
>>
> 
> 
> 
>