Subject: [PATCH] arm64: enable PREEMPT_LAZY
From: Mark Rutland <[email-masked]>
Date: Wed, 5 Mar 2025 11:49:25 +0100

For an architecture to enable CONFIG_ARCH_HAS_RESCHED_LAZY, two things are
required:
1) Adding a TIF_NEED_RESCHED_LAZY flag definition
2) Checking for TIF_NEED_RESCHED_LAZY in the appropriate locations

2) is handled in a generic manner by CONFIG_GENERIC_ENTRY, which isn't
(yet) implemented for arm64. However, outside of core scheduler code,
TIF_NEED_RESCHED_LAZY only needs to be checked on a kernel exit, meaning:
o return/entry to userspace.
o return/entry to guest.

The return/entry to a guest is all handled by xfer_to_guest_mode_handle_work()
which already does the right thing, so it can be left as-is.

arm64 doesn't use common entry's exit_to_user_mode_prepare(), so update its
return to user path to check for TIF_NEED_RESCHED_LAZY and call into
schedule() accordingly.

Link: https://lore.kernel.org/linux-rt-users/[email-masked]/
Link: https://lore.kernel.org/all/[email-masked]/
Signed-off-by: Mark Rutland <[email-masked]>
[testdrive, _TIF_WORK_MASK fixlet and changelog.]
Signed-off-by: Mike Galbraith <[email-masked]>
[Another round of testing; changelog faff]
Signed-off-by: Valentin Schneider <[email-masked]>
Reviewed-by: Sebastian Andrzej Siewior <[email-masked]>
Acked-by: Mark Rutland <[email-masked]>
Reviewed-by: Thomas Gleixner <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 arch/arm64/Kconfig                   |    1 +
 arch/arm64/include/asm/thread_info.h |   16 +++++++++-------
 arch/arm64/kernel/entry-common.c     |    2 +-
 3 files changed, 11 insertions(+), 8 deletions(-)

--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -41,6 +41,7 @@ config ARM64
 	select ARCH_HAS_NMI_SAFE_THIS_CPU_OPS
 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
 	select ARCH_HAS_NONLEAF_PMD_YOUNG if ARM64_HAFT
+	select ARCH_HAS_PREEMPT_LAZY
 	select ARCH_HAS_PTE_DEVMAP
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_HW_PTE_YOUNG
--- a/arch/arm64/include/asm/thread_info.h
+++ b/arch/arm64/include/asm/thread_info.h
@@ -59,11 +59,12 @@ void arch_setup_new_exec(void);
 
 #define TIF_SIGPENDING		0	/* signal pending */
 #define TIF_NEED_RESCHED	1	/* rescheduling necessary */
-#define TIF_NOTIFY_RESUME	2	/* callback before returning to user */
-#define TIF_FOREIGN_FPSTATE	3	/* CPU's FP state is not current's */
-#define TIF_UPROBE		4	/* uprobe breakpoint or singlestep */
-#define TIF_MTE_ASYNC_FAULT	5	/* MTE Asynchronous Tag Check Fault */
-#define TIF_NOTIFY_SIGNAL	6	/* signal notifications exist */
+#define TIF_NEED_RESCHED_LAZY	2	/* Lazy rescheduling needed */
+#define TIF_NOTIFY_RESUME	3	/* callback before returning to user */
+#define TIF_FOREIGN_FPSTATE	4	/* CPU's FP state is not current's */
+#define TIF_UPROBE		5	/* uprobe breakpoint or singlestep */
+#define TIF_MTE_ASYNC_FAULT	6	/* MTE Asynchronous Tag Check Fault */
+#define TIF_NOTIFY_SIGNAL	7	/* signal notifications exist */
 #define TIF_SYSCALL_TRACE	8	/* syscall trace active */
 #define TIF_SYSCALL_AUDIT	9	/* syscall auditing */
 #define TIF_SYSCALL_TRACEPOINT	10	/* syscall tracepoint for ftrace */
@@ -85,6 +86,7 @@ void arch_setup_new_exec(void);
 
 #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
 #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
+#define _TIF_NEED_RESCHED_LAZY	(1 << TIF_NEED_RESCHED_LAZY)
 #define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
 #define _TIF_FOREIGN_FPSTATE	(1 << TIF_FOREIGN_FPSTATE)
 #define _TIF_SYSCALL_TRACE	(1 << TIF_SYSCALL_TRACE)
@@ -100,10 +102,10 @@ void arch_setup_new_exec(void);
 #define _TIF_NOTIFY_SIGNAL	(1 << TIF_NOTIFY_SIGNAL)
 #define _TIF_TSC_SIGSEGV	(1 << TIF_TSC_SIGSEGV)
 
-#define _TIF_WORK_MASK		(_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
+#define _TIF_WORK_MASK		(_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY | \
 				 _TIF_NOTIFY_RESUME | _TIF_FOREIGN_FPSTATE | \
 				 _TIF_UPROBE | _TIF_MTE_ASYNC_FAULT | \
-				 _TIF_NOTIFY_SIGNAL)
+				 _TIF_NOTIFY_SIGNAL | _TIF_SIGPENDING)
 
 #define _TIF_SYSCALL_WORK	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
 				 _TIF_SYSCALL_TRACEPOINT | _TIF_SECCOMP | \
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -132,7 +132,7 @@ static void do_notify_resume(struct pt_r
 	do {
 		local_irq_enable();
 
-		if (thread_flags & _TIF_NEED_RESCHED)
+		if (thread_flags & (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY))
 			schedule();
 
 		if (thread_flags & _TIF_UPROBE)