Re: [PATCH 1/6] sched: decouple PREFERRED_CPU from PARAVIRT in Kconfig.preempt

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 11/2/26 7:11 PM, Ilya Leoshkevich wrote:
> 
> 
> On 11/2/26 14:00, Shrikanth Hegde wrote:
>> Hi.
>>
>>
>> On 11/2/26 5:04 PM, Ilya Leoshkevich wrote:
>>>
>>>
>>> On 11/2/26 12:00, Peter Zijlstra wrote:
>>>> On Mon, Nov 02, 2026 at 11:16:30AM +0100, Ilya Leoshkevich wrote:
>>>>>
>>>>>
>>>>> On 11/1/26 20:21, Yury Norov wrote:
>>>>>> The IBM s390 doesn't enable PARAVIRT config for their hardware
>>>>>> virtualized Logical Partitions (LPAR). However, the steal time is
>>>>>> accounted for the LPARs, once the config VIRT_CPU_ACCOUNTING_NATIVE
>>>>>> is enabled.
>>>>>>
>>>>>> Preferred cpumask doesn't rely on any feature gated by PARAVIRT config,
>>>>>> and requires it to minimize the configuration space and runtime overhead,
>>>>>> with the lack of non-paravirt user. Now that s390 wants to adopt preferred
>>>>>> cpumask, there's no reason to keep it coupled to paravirt.
>>>>>>
>>>>>> Make PREFERRED_CPU a user-configurable option, even if paravirtualization
>>>>>> is not enabled, and drop PARAVIRT dependency on the steal governor.
>>>>>>
>>>>>> Keep the default module selection conditional on PARAVIRT so other SMP
>>>>>> configurations can opt in without enabling the governor by default.
>>>>>>
>>>>>> Link: https://lore.kernel.org/all/4605dad5-aab2-49e3-b919- bf9897181834@linux.ibm.com/
>>>>>> CC: Mete Durlu <meted@linux.ibm.com>
>>>>>> Signed-off-by: Yury Norov <ynorov@nvidia.com>
>>>>>> ---
>>>>>> Can be applied independently from the rest of the series.
>>>>>>
>>>>>>    drivers/virt/Kconfig   |  8 ++++++--
>>>>>>    kernel/Kconfig.preempt | 12 ++++++++++--
>>>>>>    2 files changed, 16 insertions(+), 4 deletions(-)
>>>>>
>>>>> This is really useful to us, since even though s390 is nominally
>>>>> paravirtualized, it does not technically benefit from defining
>>>>> CONFIG_PARAVIRT.
>>>>
>>>> And therefore removing PARAVIRT is the only possible solution?
>>>>
>>>>> Specifically, steal time accounting happens by
>>>>> computing a difference between wall time and CPU time, and not by
>>>>> explicitly querying the hypervisor.
>>>>
>>>> Nobody explicitly asks the hypervisor, that would be horrendously slow.
>>>>
>>>> Something trivial like this should do.
>>>>
>>>> ---
>>>>   arch/Kconfig                           | 9 +++++++++
>>>>   arch/powerpc/platforms/pseries/Kconfig | 1 +
>>>>   arch/s390/Kconfig                      | 1 +
>>>>   drivers/virt/Kconfig                   | 2 +-
>>>>   4 files changed, 12 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/Kconfig b/arch/Kconfig
>>>> index 1163736de1f9..8980054beb52 100644
>>>> --- a/arch/Kconfig
>>>> +++ b/arch/Kconfig
>>>> @@ -1080,6 +1080,15 @@ config HAVE_IRQ_TIME_ACCOUNTING
>>>>   config HAVE_PV_STEAL_CLOCK_GEN
>>>>       bool
>>>> +#
>>>> +#  Archs that can account CPUTIME_STEAL; normal route is through PARAVIRT which
>>>> +#  selects HAVE_PV_STEAL_CLOCK_GEN. Directly selected by s390 and PPC-LPAR
>>>> +#  which have their own thing and don't go through generic cputime accounting.
>>>> +#
>>>> +config HAVE_CPUTIME_STEAL
>>>> +    bool
>>>> +    default y if HAVE_PV_STEAL_CLOCK_GEN
>>>> +
>>
>> Can't PARAVIRT_TIME_ACCOUNTING be the kconfig we can rely upon.
>> s390 can add it to the config (similar to HAVE_CPUTIME_STEAL)?
>>
>> I guess it is selected currently where the steal time reporting happens.
> 
> Looking at update_rq_clock_task(), s390 would still need to provide a
> pointless dummy paravirt_steal_clock() implementation in this case.
> 
> [...]

Ah. Peter's change seems fine to me.