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

[Ilya Leoshkevich]

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@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.
>
> 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
> +
>   config HAVE_MOVE_PUD
>   	bool
>   	help
> diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
> index 7d125e288f6e..3c49baf606d7 100644
> --- a/arch/powerpc/platforms/pseries/Kconfig
> +++ b/arch/powerpc/platforms/pseries/Kconfig
> @@ -41,6 +41,7 @@ config PPC_SPLPAR
>   	depends on PPC_PSERIES
>   	select PARAVIRT_SPINLOCKS if PPC_QUEUED_SPINLOCKS
>   	select PARAVIRT_TIME_ACCOUNTING if VIRT_CPU_ACCOUNTING_GEN
> +	select HAVE_CPUTIME_STEAL
>   	default y
>   	help
>   	  Enabling this option will make the kernel run more efficiently
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index 7138d923d86f..03a884355218 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -291,6 +291,7 @@ config S390
>   	select USER_STACKTRACE_SUPPORT
>   	select VDSO_GETRANDOM
>   	select VIRT_CPU_ACCOUNTING
> +	select HAVE_CPUTIME_STEAL
>   	select VMAP_STACK
>   	select ZONE_DMA
>   	# Note: keep the above list sorted alphabetically
> diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
> index eeb84e578ddf..a3721d34a55c 100644
> --- a/drivers/virt/Kconfig
> +++ b/drivers/virt/Kconfig
> @@ -43,7 +43,7 @@ config FSL_HV_MANAGER
>   
>   config STEAL_GOVERNOR
>   	tristate "Dynamic vCPU management based on steal time"
> -	depends on PARAVIRT && SMP
> +	depends on SMP && HAVE_CPUTIME_STEAL
>   	select PREFERRED_CPU
>   	default m
>   	help

I can't comment on the PPC part, but for s390 this works fine with the
following addition:

--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -197,4 +197,4 @@ config SCHED_CLASS_EXT

 config PREFERRED_CPU
        bool
-       depends on SMP && PARAVIRT
+       depends on SMP && HAVE_CPUTIME_STEAL