Make PFAIR optional to prevent build and runtime failures.

The PFAIR plugin always implicitly assumed !NO_HZ (the schedule
is wrong if NO_HZ is enabled) and does not built if hrtimers are absent:

> litmus/built-in.o: In function `pfair_activate_plugin':
> sched_pfair.c:(.text+0x7f07): undefined reference to `cpu_stagger_offset'
> litmus/built-in.o: In function `init_pfair':
> sched_pfair.c:(.init.text+0x487): undefined reference to `cpu_stagger_offset'

cpu_stagger_offset() is only available if hrtimers are enabled.

This patch makes these dependencies explicit.

1 files changed, 11 insertions, 0 deletions

diff --git a/litmus/Kconfig b/litmus/Kconfig
index c7cbedd1d83d..1c0a96d54992 100644
--- a/litmus/Kconfig
+++ b/litmus/Kconfig
@@ -12,6 +12,17 @@ config PLUGIN_CEDF
           On smaller platforms (e.g., ARM PB11MPCore), using C-EDF
           makes little sense since there aren't any shared caches.
 
+config PLUGIN_PFAIR
+        bool "PFAIR"
+        depends on HIGH_RES_TIMERS && !NO_HZ
+        default y
+        help
+          Include the PFAIR plugin (i.e., the PD^2 scheduler) in the kernel.
+          The PFAIR plugin requires high resolution timers (for staggered quanta)
+          and does not support NO_HZ (quanta could be missed when the system is idle).
+
+          If unsure, say Yes.
+
 config RELEASE_MASTER
         bool "Release-master Support"
         depends on ARCH_HAS_SEND_PULL_TIMERS