[PATCH v2] sched/rt: Disable RT_RUNTIME_SHARE by default

[Daniel Bristot de Oliveira]

The RT_RUNTIME_SHARE sched feature enables the sharing of rt_runtime
between CPUs, allowing a CPU to run a real-time task up to 100% of the
time while leaving more space for non-real-time tasks to run on the CPU
that lend rt_runtime.

The problem is that a CPU can easily borrow enough rt_runtime to allow
a spinning rt-task to run forever, starving per-cpu tasks like kworkers,
which are non-real-time by design.

This patch disables RT_RUNTIME_SHARE by default, avoiding this problem.
The feature will still be present for users that want to enable it,
though. It also documents this option.

v1->v2:
 Unified the documentation and the disabling in a single patch

Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Clark Williams <williams@xxxxxxxxxx>
Cc: Luis Claudio R. Goncalves <lgoncalv@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Acked-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Acked-by: Luis Claudio R. Goncalves <lgoncalv@xxxxxxxxxx>

diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 69631fa..d609ca9 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -65,7 +65,17 @@ SCHED_FEAT(RT_PUSH_IPI, true)
 #endif
 
 SCHED_FEAT(FORCE_SD_OVERLAP, false)
-SCHED_FEAT(RT_RUNTIME_SHARE, true)
+
+/*
+ * Enables the sharing of rt_runtime between CPUs, allowing a CPU to
+ * run real-time tasks up to 100% of the time while leaving more
+ * space for non-real-time tasks to run on the CPU that lend rt_runtime.
+ *
+ * WARNING: This may allow the starvation of non-real-time tasks pinned
+ * to the CPU in which spinning rt-tasks run forever.
+ */
+SCHED_FEAT(RT_RUNTIME_SHARE, false)
+
 SCHED_FEAT(LB_MIN, false)
 SCHED_FEAT(ATTACH_AGE_LOAD, true)
 
-- 
2.7.4