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

[Ilya Leoshkevich <iii@linux.ibm.com>]

On 11/3/26 06:49, Shrikanth Hegde wrote:
> Hi Ilya,
> 
> 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.
>>
>> [...]
> 
> Other possible option is, you could use CONFIG_HAVE_PV_STEAL_CLOCK_GEN 
> which
> defines the dummy implementation and you don't update that method. That way
> you may implement PARAVIRT_TIME_ACCOUNTING mostly with kconfig changes.
> 
> But it needs the arch to implement static calls. I am not sure if s390 
> has it.

We will get non-optimized static calls soon [1], so this would be not so
great from performance PoV.

I think HAVE_CPUTIME_STEAL is a good solution. It allows to disable
PREFERRED_CPU and STEAL_GOVERNOR on systems which do not do steal time
accounting, while, unlike PARAVIRT, PARAVIRT_TIME_ACCOUNTING, and
HAVE_PV_STEAL_CLOCK_GEN, not prescribing how the other architectures
should implement steal time accounting.

[1] 
https://lore.kernel.org/linux-s390/20261026102952.4153711-1-hca@linux.ibm.com/