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Define get/set macros > > > > which could be used to get/set CPU state as preferred. > > > > > > > > CONFIG_PREFERRED_CPU will be selected by the driver which handles > > > > steal time values. It is going to set/clear preferred CPU state. > > > > This driver will be called steal_governor and it is introduced in > > > > subsequent patches. It periodically computes the steal ratio and > > > > decides on preferred CPU state. > > > > > > > > A CPU is set to preferred when it becomes active. Later it may be > > > > marked as non-preferred depending on steal ratio by the steal_governor. > > > > > > > > Always maintain design construct of preferred is subset of active. > > > > i.e. preferred ⊆ active ⊆ online ⊆ present ⊆ possible > > > > > > > > With CONFIG_PREFERRED_CPU=n, ensure set_cpu_preferred is a nop and get > > > > method returns the active state in that case. > > > > > > > > Signed-off-by: Shrikanth Hegde > > > > --- > > > > include/linux/cpumask.h | 24 ++++++++++++++++++++++++ > > > > kernel/Kconfig.preempt | 4 ++++ > > > > kernel/cpu.c | 6 ++++++ > > > > kernel/sched/core.c | 5 +++++ > > > > 4 files changed, 39 insertions(+) > > > > > > [..snip..] > > > > > > > #define cpu_is_offline(cpu) unlikely(!cpu_online(cpu)) > > > > diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt > > > > index 985aea617cfe..edc067a0c422 100644 > > > > --- a/kernel/Kconfig.preempt > > > > +++ b/kernel/Kconfig.preempt > > > > @@ -194,3 +194,7 @@ config SCHED_CLASS_EXT > > > > For more information: > > > > Documentation/scheduler/sched-ext.rst > > > > https://github.com/sched-ext/scx > > > > + > > > > +config PREFERRED_CPU > > > > + bool > > > > + depends on SMP && PARAVIRT > > > FWICT, PREFERRED_CPU mechanism can work perfectly fine without PARAVIRT > > > selected. It would be annoying to enable it and implement a dummy > > > paravirt_steal_clock() just for that. > > > > > > Shrikanth, would it be possible to drop that dependency? > > > > Do you have a non-paravirt case in mind? > > s390 LPARs are exactly that. s390 does not select PARAVIRT by default > but enabling PREFERRED_CPU is strongly considered. > > I removed the PARAVIRT dependency and gave the whole thing a quick > spin. Builds and functions w/o any issues on s390! > > I'm actually about to submit an RFC that drops the PARAVIRT dependency > > and provides write access to the preferred mask. But I want to make > > sure that we need it, and we've got no reasonable mechanism to put > > unneeded CPUs on idle already. > > You can mention s390 LPARs as valid case on your RFC if need be. Sure, will do. I'm about submitting this before the end of week; will add you in CC. Thanks, Yury