[PATCH 5.4] sched/rt: Disable RT_RUNTIME_SHARE by default

[PATCH 5.4] sched/rt: Disable RT_RUNTIME_SHARE by default

[Mark-PK Tsai]

From: Daniel Bristot de Oliveira <bristot@redhat.com>

commit 2586af1ac187f6b3a50930a4e33497074e81762d upstream.

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@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Wei Wang <wvw@google.com>
Link: https://lkml.kernel.org/r/b776ab46817e3db5d8ef79175fa0d71073c051c7.1600697903.git.bristot@redhat.com
Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
Cc: stable@vger.kernel.org
---
 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 2410db5e9a35..66c74aa4753e 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)
 
-- 
2.18.0

Re: [PATCH 5.4] sched/rt: Disable RT_RUNTIME_SHARE by default

[Greg KH]

On Thu, Jul 14, 2022 at 03:30:52PM +0800, Mark-PK Tsai wrote:
> From: Daniel Bristot de Oliveira <bristot@redhat.com>
> 
> commit 2586af1ac187f6b3a50930a4e33497074e81762d upstream.
> 
> 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@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Tested-by: Wei Wang <wvw@google.com>
> Link: https://lkml.kernel.org/r/b776ab46817e3db5d8ef79175fa0d71073c051c7.1600697903.git.bristot@redhat.com
> Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
> Cc: stable@vger.kernel.org
> ---
>  kernel/sched/features.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Now queued up, thanks.

greg k-h