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

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Message-ID: <457e8ca1-beb3-ca39-b257-e7bc6bb35d4d@oracle.com>
Date:   Tue, 3 Sep 2019 11:13:31 +0100
From:   Joao Martins <joao.m.martins@...cle.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        kvm-devel <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <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>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Linux PM <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 9/2/19 10:55 PM, Rafael J. Wysocki wrote:
> On Thu, Aug 29, 2019 at 7:24 PM Marcelo Tosatti <mtosatti@...hat.com> wrote:
>>
>> On Thu, Aug 29, 2019 at 06:16:05PM +0100, 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.
>>
>>> Right now, for a KVM guest, the
>>> idle governors have these ratings:
>>>
>>>  * ladder            -> 10
>>>  * teo               -> 19
>>>  * menu              -> 20
>>>  * haltpoll          -> 21
>>>  * ladder + nohz=off -> 25
>>
>> Yes. PowerPC KVM guests crash currently due to the use of the haltpoll
>> governor (have a patch in my queue to fix this, but your solution
>> embraces more cases).
>>
>>> 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 modloads.
>>>
>>> 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=.
>>>
>>> The diff below the scissors line is an example of that.
>>>
>>> Thoughts?
>>
>> Works for me. Rafael?
> 
> It works for me too, basically, except that I would rename
> cpuidle_default_governor in the patch to cpuidle_prev_governor.
> 

Great! I'll send over a series with this then (splitted accordingly).

Also, In the course of hotplug/hotunplug testing, I found two small issues with
modload/modunload -- regardless of the hotplug patch. So I am gonna add that to
the series too.

	Joao