Re: [PATCH] sched: Decouple PREFERRED_CPU from PARAVIRT

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

On 11/6/26 6:05 PM, Mete Durlu wrote:
> PREFERRED_CPU mechanism does not rely on any feature introduced by PARAVIRT
> but PREFERRED_CPU still gated behind PARAVIRT. Any architecture that
> would like to enable PREFERRED_CPU mechanism has to also enable PARAVIRT
> and bring additional changes to accommodate PARAVIRT's requirements.
> 
> As an example, s390 doesn't enable PARAVIRT config for hardware virtualized
> Logical Partitions (LPAR). The steal time is accounted for the LPARs via
> architecture's own steal time accounting mechanism instead of the common
> steal time infrastructure PARAVIRT supplies.
> For s390 to enable PREFERRED_CPU, PRAVIRT and its required stubs are
> just unncessary.
> 
> Based on the discussion between Yury Norov and Peter Zijlstra over
> Yury's original patch [1]:
> 
> Introduce a new config flag HAVE_CPUTIME_STEAL to indicate that the
> architecture has steal time measurement capabilities already implemented
> and to signal that PARAVIRT is not required.
> Remove PARAVIRT gating from CPU_PREFERRED and add HAVE_CPUTIME_STEAL
> instead.
> Select HAVE_CPUTIME_STEAL for s390 and powerpc-pseries configs.
> 
> [1] https://lore.kernel.org/all/20261101192203.6979-2-ynorov@nvidia.com/
> 
> Reviewed-by: Yury Norov <ynorov@nvidia.com>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Mete Durlu <meted@linux.ibm.com>

Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>

> ---
>   arch/Kconfig                           | 8 ++++++++
>   arch/powerpc/platforms/pseries/Kconfig | 1 +
>   arch/s390/Kconfig                      | 1 +
>   drivers/virt/Kconfig                   | 2 +-
>   kernel/Kconfig.preempt                 | 2 +-
>   5 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 1163736de1f9..01b8251970f6 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -1080,6 +1080,14 @@ config HAVE_IRQ_TIME_ACCOUNTING
>   config HAVE_PV_STEAL_CLOCK_GEN
>   	bool
>   
> +config HAVE_CPUTIME_STEAL
> +	bool
> +	default y if HAVE_PV_STEAL_CLOCK_GEN
> +	help
> +	  Selected directly by config PARAVIRT through HAVE_PV_STEAL_CLOCK_GEN
> +	  or directly by architectures that have their own CPUTIME_STEAL
> +	  accounting.
> +
>   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..39379e72ffdf 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -204,6 +204,7 @@ config S390
>   	select HAVE_BUILDTIME_MCOUNT_SORT
>   	select HAVE_CMPXCHG_DOUBLE
>   	select HAVE_CMPXCHG_LOCAL
> +	select HAVE_CPUTIME_STEAL
>   	select HAVE_DEBUG_KMEMLEAK
>   	select HAVE_DMA_CONTIGUOUS
>   	select HAVE_DYNAMIC_FTRACE
> 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
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index edc067a0c422..ffd2cb2a4d69 100644
> --- 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
> 
> ---
> base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
> change-id: 20261104-preferredcpu-b3c00594bd74
> 
> Best regards,