Subject: arm: Add support for lazy preemption
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed Oct 31 12:04:11 2012 +0100

From: Thomas Gleixner <tglx@linutronix.de>

Implement the arm pieces for lazy preempt.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>


---
 arch/arm/Kconfig                   |    1 +
 arch/arm/include/asm/thread_info.h |    6 +++++-
 arch/arm/kernel/asm-offsets.c      |    1 +
 arch/arm/kernel/entry-armv.S       |   19 ++++++++++++++++---
 arch/arm/kernel/signal.c           |    3 ++-
 5 files changed, 25 insertions(+), 5 deletions(-)
---
Index: linux-6.3.0-rt11/arch/arm/Kconfig
===================================================================
@ linux-6.3.0-rt11/arch/arm/Kconfig:120 @ config ARM
 	select HAVE_PERF_EVENTS
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
+	select HAVE_PREEMPT_LAZY
 	select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RSEQ
Index: linux-6.3.0-rt11/arch/arm/include/asm/thread_info.h
===================================================================
--- linux-6.3.0-rt11.orig/arch/arm/include/asm/thread_info.h
+++ linux-6.3.0-rt11/arch/arm/include/asm/thread_info.h
@ linux-6.3.0-rt11/arch/arm/Kconfig:65 @ struct cpu_context_save {
 struct thread_info {
 	unsigned long		flags;		/* low level flags */
 	int			preempt_count;	/* 0 => preemptable, <0 => bug */
+	int			preempt_lazy_count; /* 0 => preemptable, <0 => bug */
 	__u32			cpu;		/* cpu */
 	__u32			cpu_domain;	/* cpu domain */
 	struct cpu_context_save	cpu_context;	/* cpu context */
@ linux-6.3.0-rt11/arch/arm/Kconfig:133 @ extern int vfp_restore_user_hwstate(stru
 #define TIF_NOTIFY_RESUME	2	/* callback before returning to user */
 #define TIF_UPROBE		3	/* breakpointed or singlestepping */
 #define TIF_NOTIFY_SIGNAL	4	/* signal notifications exist */
+#define TIF_NEED_RESCHED_LAZY	5
 
 #define TIF_USING_IWMMXT	17
 #define TIF_MEMDIE		18	/* is terminating due to OOM killer */
@ linux-6.3.0-rt11/arch/arm/Kconfig:153 @ extern int vfp_restore_user_hwstate(stru
 #define _TIF_SYSCALL_TRACEPOINT	(1 << TIF_SYSCALL_TRACEPOINT)
 #define _TIF_SECCOMP		(1 << TIF_SECCOMP)
 #define _TIF_NOTIFY_SIGNAL	(1 << TIF_NOTIFY_SIGNAL)
+#define _TIF_NEED_RESCHED_LAZY	(1 << TIF_NEED_RESCHED_LAZY)
 #define _TIF_USING_IWMMXT	(1 << TIF_USING_IWMMXT)
 
 /* Checks for any syscall work in entry-common.S */
@ linux-6.3.0-rt11/arch/arm/Kconfig:163 @ extern int vfp_restore_user_hwstate(stru
 /*
  * Change these and you break ASM code in entry-common.S
  */
-#define _TIF_WORK_MASK		(_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
+#define _TIF_WORK_MASK		(_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY | \
+				 _TIF_SIGPENDING | \
 				 _TIF_NOTIFY_RESUME | _TIF_UPROBE | \
 				 _TIF_NOTIFY_SIGNAL)
 
Index: linux-6.3.0-rt11/arch/arm/kernel/asm-offsets.c
===================================================================
--- linux-6.3.0-rt11.orig/arch/arm/kernel/asm-offsets.c
+++ linux-6.3.0-rt11/arch/arm/kernel/asm-offsets.c
@ linux-6.3.0-rt11/arch/arm/Kconfig:46 @ int main(void)
   BLANK();
   DEFINE(TI_FLAGS,		offsetof(struct thread_info, flags));
   DEFINE(TI_PREEMPT,		offsetof(struct thread_info, preempt_count));
+  DEFINE(TI_PREEMPT_LAZY,	offsetof(struct thread_info, preempt_lazy_count));
   DEFINE(TI_CPU,		offsetof(struct thread_info, cpu));
   DEFINE(TI_CPU_DOMAIN,		offsetof(struct thread_info, cpu_domain));
   DEFINE(TI_CPU_SAVE,		offsetof(struct thread_info, cpu_context));
Index: linux-6.3.0-rt11/arch/arm/kernel/entry-armv.S
===================================================================
--- linux-6.3.0-rt11.orig/arch/arm/kernel/entry-armv.S
+++ linux-6.3.0-rt11/arch/arm/kernel/entry-armv.S
@ linux-6.3.0-rt11/arch/arm/Kconfig:225 @ __irq_svc:
 
 #ifdef CONFIG_PREEMPTION
 	ldr	r8, [tsk, #TI_PREEMPT]		@ get preempt count
-	ldr	r0, [tsk, #TI_FLAGS]		@ get flags
 	teq	r8, #0				@ if preempt count != 0
+	bne	1f				@ return from exeption
+	ldr	r0, [tsk, #TI_FLAGS]		@ get flags
+	tst	r0, #_TIF_NEED_RESCHED		@ if NEED_RESCHED is set
+	blne	svc_preempt			@ preempt!
+
+	ldr	r8, [tsk, #TI_PREEMPT_LAZY]	@ get preempt lazy count
+	teq	r8, #0				@ if preempt lazy count != 0
 	movne	r0, #0				@ force flags to 0
-	tst	r0, #_TIF_NEED_RESCHED
+	tst	r0, #_TIF_NEED_RESCHED_LAZY
 	blne	svc_preempt
+1:
 #endif
 
 	svc_exit r5, irq = 1			@ return from exception
@ linux-6.3.0-rt11/arch/arm/Kconfig:251 @ svc_preempt:
 1:	bl	preempt_schedule_irq		@ irq en/disable is done inside
 	ldr	r0, [tsk, #TI_FLAGS]		@ get new tasks TI_FLAGS
 	tst	r0, #_TIF_NEED_RESCHED
+	bne	1b
+	tst	r0, #_TIF_NEED_RESCHED_LAZY
 	reteq	r8				@ go again
-	b	1b
+	ldr	r0, [tsk, #TI_PREEMPT_LAZY]	@ get preempt lazy count
+	teq	r0, #0				@ if preempt lazy count != 0
+	beq	1b
+	ret	r8				@ go again
+
 #endif
 
 __und_fault:
Index: linux-6.3.0-rt11/arch/arm/kernel/signal.c
===================================================================
--- linux-6.3.0-rt11.orig/arch/arm/kernel/signal.c
+++ linux-6.3.0-rt11/arch/arm/kernel/signal.c
@ linux-6.3.0-rt11/arch/arm/Kconfig:610 @ do_work_pending(struct pt_regs *regs, un
 	 */
 	trace_hardirqs_off();
 	do {
-		if (likely(thread_flags & _TIF_NEED_RESCHED)) {
+		if (likely(thread_flags & (_TIF_NEED_RESCHED |
+					   _TIF_NEED_RESCHED_LAZY))) {
 			schedule();
 		} else {
 			if (unlikely(!user_mode(regs)))