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

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Date:   Thu, 29 Aug 2019 19:42:05 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Joao Martins <joao.m.martins@...cle.com>,
        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 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?


> 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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