Re: Lazy preemption on arm64

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Dec 16, 2024 at 07:04:51PM +0100, Petr Tesarik wrote:
> Hi all,
> 
> what is the plan for implementing PREEMPT_LAZY on arm64?
> 
> There used to be RT patch series which enabled lazy preemption on
> arm64, but this architecture was "sacrificed" in v6.6-rc6-rt10, as
> collateral damage of switching to PREEMPT_AUTO.
> 
> IIUC lazy preemption is currently implemented only for architectures
> with CONFIG_GENERIC_ENTRY, but there is no inherent dependency on it.
> So, is the plan to convert arm64 to GENERIC_ENTRY (and then get
> PREEMPT_LAZY for free), or is somebody working on CONFIG_PREEMPT_LAZY
> for arm64 without that conversion?

I don't think there's an agreed upon plan either way.

Jinjie Ruan has been looking to move arm64 over to GENERIC_ENTRY:

  https://lore.kernel.org/all/20241206101744.4161990-1-ruanjinjie@huawei.com/

AFAICT, the only bits that we get "for free" from GENERIC_ENTRY would be
the logic in raw_irqentry_exit_cond_resched() and
exit_to_user_mode_loop(), and all we'd need to enable this on arm64
as-is would be as below.

... so how important is this?

Mark.

---->8----
From 16ae3c84d8e1691fe670dbfb4c643cab4fa2065f Mon Sep 17 00:00:00 2001
From: Mark Rutland <mark.rutland@arm.com>
Date: Mon, 16 Dec 2024 18:32:44 +0000
Subject: [PATCH] HACK: arm64: enable PREEMPT_LAZY

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
---
 arch/arm64/Kconfig                   |  1 +
 arch/arm64/include/asm/thread_info.h | 12 +++++++-----
 arch/arm64/kernel/entry-common.c     |  2 +-
 3 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 100570a048c5e..7926bc78a1c46 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -39,6 +39,7 @@ 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
diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
index 1114c1c3300a1..a2125407901ed 100644
--- 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)
diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index b260ddc4d3e9a..7993fab0cab4c 100644
--- 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_regs *regs, unsigned long thread_flags)
 	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)
-- 
2.30.2