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

[Peter Zijlstra]

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