On 12/16/24 14:05, Mark Rutland wrote:
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@xxxxxxxxxx/
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?
For running machine tools with LinuxCNC, very. I have an rpi4b running
a medium sized Sheldon lathe thats all of 80 yo and its still using a
4.19-rt kernel with quite low for a pi, latency. But its 10x slower
than wintel stuff.
Mark.
---->8----
From 16ae3c84d8e1691fe670dbfb4c643cab4fa2065f Mon Sep 17 00:00:00 2001
From: Mark Rutland <mark.rutland@xxxxxxx>
Date: Mon, 16 Dec 2024 18:32:44 +0000
Subject: [PATCH] HACK: arm64: enable PREEMPT_LAZY
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
---
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)
Cheers, Gene Heskett, CET.
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
- Louis D. Brandeis
