linux-kernel - Re: [PATCH 0/2] pseries/hotplug: Change the default behaviour of cede

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Date:   Wed, 18 Sep 2019 15:17:19 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nathan Lynch <nathanl@...ux.ibm.com>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Kamalesh Babulal <kamaleshb@...ibm.com>,
        "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Tyrel Datwyler <tyreld@...ux.ibm.com>
Subject: Re: [PATCH 0/2] pseries/hotplug: Change the default behaviour of cede_offline

Nathan Lynch <nathanl@...ux.ibm.com> writes:
> Gautham R Shenoy <ego@...ux.vnet.ibm.com> writes:
>> On Thu, Sep 12, 2019 at 10:39:45AM -0500, Nathan Lynch wrote:
>>> "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> writes:
>>> > The patchset also defines a new sysfs attribute
>>> > "/sys/device/system/cpu/cede_offline_enabled" on PSeries Linux guests
>>> > to allow userspace programs to change the state into which the
>>> > offlined CPU need to be put to at runtime.
>>> 
>>> A boolean sysfs interface will become awkward if we need to add another
>>> mode in the future.
>>> 
>>> What do you think about naming the attribute something like
>>> 'offline_mode', with the possible values 'extended-cede' and
>>> 'rtas-stopped'?
>>
>> We can do that. However, IMHO in the longer term, on PSeries guests,
>> we should have only one offline state - rtas-stopped.  The reason for
>> this being, that on Linux, SMT switch is brought into effect through
>> the CPU Hotplug interface. The only state in which the SMT switch will
>> recognized by the hypervisors such as PHYP is rtas-stopped.
>
> OK. Why "longer term" though, instead of doing it now?
>
>
>> All other states (such as extended-cede) should in the long-term be
>> exposed via the cpuidle interface.
>>
>> With this in mind, I made the sysfs interface boolean to mirror the
>> current "cede_offline" commandline parameter. Eventually when we have
>> only one offline-state, we can deprecate the commandline parameter as
>> well as the sysfs interface.
>
> I don't care for adding a sysfs interface that is intended from the
> beginning to become vestigial...
>
> This strikes me as unnecessarily incremental if you're changing the
> default offline state. Any user space programs depending on the current
> behavior will have to change anyway (and why is it OK to break them?)
>
> Why isn't the plan:
>
>   1. Add extended cede support to the pseries cpuidle driver
>   2. Make stop-self the only cpu offline state for pseries (no sysfs
>      interface necessary)

I agree, that would be preferable. Adding more sysfs tunables sucks,
especially if they're going to be deprecated in the not too distant
future.

Another option would be to add extended cede to the cpuidle driver,
and retain the cede_offline boot parameter but make it false by default.

cheers