[PATCH 0/2] arm: Disable two features which are not compatible with PREEMPT_RT

[PATCH 0/2] arm: Disable two features which are not compatible with PREEMPT_RT

[Sebastian Andrzej Siewior]

Hi,

this mini series disables two features on PREEMPT_RT which are not
compatible with PREEMPT_RT. The softirq stacks are also disabled on x86.
The jump-labels could be enabled if stop_machine() is avoided.

Sebastian



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[PATCH 1/2] arm: Disable jump-label on PREEMPT_RT.

[Sebastian Andrzej Siewior]

From: Thomas Gleixner <tglx@linutronix.de>

jump-labels are used to efficiently switch between two possible code
paths. To achieve this, stop_machine() is used to keep the CPU in a
known state while the opcode is modified. The usage of stop_machine()
here leads to large latency spikes which can be observed on PREEMPT_RT.

Jump labels may change the target during runtime and are not restricted
to debug or "configuration/ setup" part of a PREEMPT_RT system where
high latencies could be defined as acceptable.

Disable jump-label support on a PREEMPT_RT system.

[bigeasy: Patch description.]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 arch/arm/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 7630ba9cb6ccc..2088d9322588e 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -71,7 +71,7 @@ config ARM
 	select HARDIRQS_SW_RESEND
 	select HAVE_ARCH_AUDITSYSCALL if AEABI && !OABI_COMPAT
 	select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
-	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU
+	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU && !PREEMPT_RT
 	select HAVE_ARCH_KFENCE if MMU && !XIP_KERNEL
 	select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32 && MMU
 	select HAVE_ARCH_KASAN if MMU && !XIP_KERNEL
-- 
2.36.1


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[PATCH 2/2] arm: Disable softirq stacks on PREEMPT_RT.

[Sebastian Andrzej Siewior]

PREEMPT_RT preempts softirqs and the current implementation avoids
do_softirq_own_stack() and only uses __do_softirq().

Disable the unused softirqs stacks on PREEMPT_RT to safe some memory and
ensure that do_softirq_own_stack() is not used which is not expected.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 arch/arm/kernel/irq.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm/kernel/irq.c b/arch/arm/kernel/irq.c
index 5c6f8d11a3ce5..034cb48c9eeb8 100644
--- a/arch/arm/kernel/irq.c
+++ b/arch/arm/kernel/irq.c
@@ -70,6 +70,7 @@ static void __init init_irq_stacks(void)
 	}
 }
 
+#ifndef CONFIG_PREEMPT_RT
 static void ____do_softirq(void *arg)
 {
 	__do_softirq();
@@ -80,7 +81,7 @@ void do_softirq_own_stack(void)
 	call_with_stack(____do_softirq, NULL,
 			__this_cpu_read(irq_stack_ptr));
 }
-
+#endif
 #endif
 
 int arch_show_interrupts(struct seq_file *p, int prec)
-- 
2.36.1


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