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

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

On 11/3/26 12:26, Ilya Leoshkevich wrote:
> 
> 
> 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 just realized it's enabled only when CFI is enabled, so it's not an
option anyway.

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