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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Aug 29, 2019 at 7:24 PM Marcelo Tosatti <mtosatti@redhat.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@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.
>
> > 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.