From 256d97d3587b441630c345ab3d773a9e7dcd964b Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Tue, 11 Nov 2025 16:54:36 +0100 Subject: [PATCH 1/2] ARM: 9459/1: Disable jump-label on PREEMPT_RT 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. On 64-bit Arm, it is possible to use jump labels without the stop_machine() call, which architecturally provides a way to atomically change one 32-bit instruction word while keeping maintaining consistency, but this is not generally the case on 32-bit, in particular in thumb2 mode. Disable jump-label support on a PREEMPT_RT system when SMP is enabled. [bigeasy: Patch description.] [arnd: add !SMP case, extend changelog] Signed-off-by: Thomas Gleixner Cc: Mark Rutland Cc: Ard Biesheuvel Signed-off-by: Arnd Bergmann Reviewed-by: Linus Walleij Acked-by: Ard Biesheuvel Acked-by: Linus Walleij Reviewed-by: Arnd Bergmann Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Russell King (Oracle) --- arch/arm/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig index 2e3f93b690f4..92ca24336ec4 100644 --- a/arch/arm/Kconfig +++ b/arch/arm/Kconfig @@ -80,7 +80,7 @@ config ARM select HAS_IOPORT 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 || !SMP) 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 From fedadc4137234c3d00c4785eeed3e747fe9036ae Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Tue, 11 Nov 2025 16:54:37 +0100 Subject: [PATCH 2/2] ARM: 9461/1: Disable HIGHPTE on PREEMPT_RT kernels gup_pgd_range() is invoked with disabled interrupts and invokes __kmap_local_page_prot() via pte_offset_map(), gup_p4d_range(). With HIGHPTE enabled, __kmap_local_page_prot() invokes kmap_high_get() which uses a spinlock_t via lock_kmap_any(). This leads to an sleeping-while-atomic error on PREEMPT_RT because spinlock_t becomes a sleeping lock and must not be acquired in atomic context. The loop in map_new_virtual() uses wait_queue_head_t for wake up which also is using a spinlock_t. Since HIGHPTE is rarely needed at all, turn it off for PREEMPT_RT to allow the use of get_user_pages_fast(). [arnd: rework patch to turn off HIGHPTE instead of HAVE_PAST_GUP] Co-developed-by: Arnd Bergmann Acked-by: Linus Walleij Reviewed-by: Arnd Bergmann Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Russell King (Oracle) --- arch/arm/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig index 92ca24336ec4..45d29320196f 100644 --- a/arch/arm/Kconfig +++ b/arch/arm/Kconfig @@ -1213,7 +1213,7 @@ config HIGHMEM config HIGHPTE bool "Allocate 2nd-level pagetables from highmem" if EXPERT - depends on HIGHMEM + depends on HIGHMEM && !PREEMPT_RT default y help The VM uses one page of physical memory for each page table.