sched: Cleanup SCHED_THERMAL_PRESSURE kconfig entry

As Russell pointed out [1], this option is severely lacking in the
documentation department, and figuring out if one has the required
dependencies to benefit from turning it on is not straightforward.

Make it non user-visible, and add a bit of help to it. While at it, make it
depend on CPU_FREQ_THERMAL.

[1]: https://lkml.kernel.org/r/20200603173150.GB1551@shell.armlinux.org.ukSigned-off-by: NValentin Schneider <valentin.schneider@arm.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200712165917.9168-3-valentin.schneider@arm.com

init/Kconfig

@@ -492,8 +492,21 @@ config HAVE_SCHED_AVG_IRQ
 	depends on SMP
 
 config SCHED_THERMAL_PRESSURE
-	bool "Enable periodic averaging of thermal pressure"
+	bool
 	depends on SMP
+	depends on CPU_FREQ_THERMAL
+	help
+	  Select this option to enable thermal pressure accounting in the
+	  scheduler. Thermal pressure is the value conveyed to the scheduler
+	  that reflects the reduction in CPU compute capacity resulted from
+	  thermal throttling. Thermal throttling occurs when the performance of
+	  a CPU is capped due to high operating temperatures.
+
+	  If selected, the scheduler will be able to balance tasks accordingly,
+	  i.e. put less load on throttled CPUs than on non/less throttled ones.
+
+	  This requires the architecture to implement
+	  arch_set_thermal_pressure() and arch_get_thermal_pressure().
 
 config BSD_PROCESS_ACCT
 	bool "BSD Process Accounting"