[tip: sched/core] sched/rt: Disable RT_RUNTIME_SHARE by default

[tip-bot2 for Daniel Bristot de Oliveira]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     2586af1ac187f6b3a50930a4e33497074e81762d
Gitweb:        https://git.kernel.org/tip/2586af1ac187f6b3a50930a4e33497074e81762d
Author:        Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
AuthorDate:    Mon, 21 Sep 2020 16:39:49 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 25 Sep 2020 14:23:24 +02:00

sched/rt: Disable RT_RUNTIME_SHARE by default

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.

Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Wei Wang <wvw@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/b776ab46817e3db5d8ef79175fa0d71073c051c7.1600697903.git.bristot@xxxxxxxxxx
---
 kernel/sched/features.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 7481cd9..68d369c 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -77,7 +77,7 @@ SCHED_FEAT(WARN_DOUBLE_CLOCK, false)
 SCHED_FEAT(RT_PUSH_IPI, true)
 #endif
 
-SCHED_FEAT(RT_RUNTIME_SHARE, true)
+SCHED_FEAT(RT_RUNTIME_SHARE, false)
 SCHED_FEAT(LB_MIN, false)
 SCHED_FEAT(ATTACH_AGE_LOAD, true)