[PATCH v4 1/1] powerpc: enable dynamic preemption

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Once the lazy preemption is supported, it would be desirable to change
the preemption models at runtime. So add support for dynamic preemption
using DYNAMIC_KEY.

::Tested lightly on Power10 LPAR
Performance numbers indicate that, preempt=none(no dynamic) and
preempt=none(dynamic) are close.

cat /sys/kernel/debug/sched/preempt
(none) voluntary full lazy
perf stat -e probe:__cond_resched -a sleep 1
 Performance counter stats for 'system wide':
             1,253      probe:__cond_resched

echo full > /sys/kernel/debug/sched/preempt
cat /sys/kernel/debug/sched/preempt
none voluntary (full) lazy
perf stat -e probe:__cond_resched -a sleep 1
 Performance counter stats for 'system wide':
                 0      probe:__cond_resched

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 arch/powerpc/Kconfig               |  1 +
 arch/powerpc/include/asm/preempt.h | 16 ++++++++++++++++
 arch/powerpc/kernel/interrupt.c    |  6 +++++-
 arch/powerpc/lib/vmx-helper.c      |  2 +-
 4 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 arch/powerpc/include/asm/preempt.h

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 424f188e62d9..364edaea1f88 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -275,6 +275,7 @@ config PPC
 	select HAVE_PERF_EVENTS_NMI		if PPC64
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
+	select HAVE_PREEMPT_DYNAMIC_KEY
 	select HAVE_RETHOOK			if KPROBES
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RELIABLE_STACKTRACE
diff --git a/arch/powerpc/include/asm/preempt.h b/arch/powerpc/include/asm/preempt.h
new file mode 100644
index 000000000000..000e2b9681f3
--- /dev/null
+++ b/arch/powerpc/include/asm/preempt.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_POWERPC_PREEMPT_H
+#define __ASM_POWERPC_PREEMPT_H
+
+#include <asm-generic/preempt.h>
+
+#if defined(CONFIG_PREEMPT_DYNAMIC)
+#include <linux/jump_label.h>
+DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
+#define need_irq_preemption() \
+	(static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
+#else
+#define need_irq_preemption()   (IS_ENABLED(CONFIG_PREEMPTION))
+#endif
+
+#endif /* __ASM_POWERPC_PREEMPT_H */
diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 8f4acc55407b..e0c681d0b076 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -25,6 +25,10 @@
 unsigned long global_dbcr0[NR_CPUS];
 #endif
 
+#if defined(CONFIG_PREEMPT_DYNAMIC)
+DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
+#endif
+
 #ifdef CONFIG_PPC_BOOK3S_64
 DEFINE_STATIC_KEY_FALSE(interrupt_exit_not_reentrant);
 static inline bool exit_must_hard_disable(void)
@@ -396,7 +400,7 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
 		/* Returning to a kernel context with local irqs enabled. */
 		WARN_ON_ONCE(!(regs->msr & MSR_EE));
 again:
-		if (IS_ENABLED(CONFIG_PREEMPTION)) {
+		if (need_irq_preemption()) {
 			/* Return to preemptible kernel context */
 			if (unlikely(read_thread_flags() & _TIF_NEED_RESCHED)) {
 				if (preempt_count() == 0)
diff --git a/arch/powerpc/lib/vmx-helper.c b/arch/powerpc/lib/vmx-helper.c
index 58ed6bd613a6..54340912398f 100644
--- a/arch/powerpc/lib/vmx-helper.c
+++ b/arch/powerpc/lib/vmx-helper.c
@@ -45,7 +45,7 @@ int exit_vmx_usercopy(void)
 	 * set and we are preemptible. The hack here is to schedule a
 	 * decrementer to fire here and reschedule for us if necessary.
 	 */
-	if (IS_ENABLED(CONFIG_PREEMPTION) && need_resched())
+	if (need_irq_preemption() && need_resched())
 		set_dec(1);
 	return 0;
 }
-- 
2.39.3