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

[Ilya Leoshkevich]

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@xxxxxxxxxxxxx/
> CC: Mete Durlu <meted@xxxxxxxxxxxxx>
> Signed-off-by: Yury Norov <ynorov@xxxxxxxxxx>
> ---
> 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. Specifically, steal time accounting happens by
computing a difference between wall time and CPU time, and not by
explicitly querying the hypervisor.

Thanks!

Reviewed-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>

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