[PATCH 6/6] arm64: support PREEMPT_DYNAMIC

Mark Rutland
2021-11-09T17:26:44Z

This patch enables support for PREEMPT_DYNAMIC on arm64, using static
keys to disable the out-of-line forms of the preemption functions. This
side-steps a number of pain points highlighted with static calls, and I
expect the overhead to be similar (and hopefully lower in some cases)
than would be the case for non-inlined static calls, by virtue of
effectively inlining the trampolines into the callees.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
---
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/preempt.h | 16 ++++++++++++++--
arch/arm64/kernel/entry-common.c | 9 +++++++++
3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index c4207cf9bb17..928b710465ce 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -191,6 +191,7 @@ config ARM64
select HAVE_PERF_EVENTS
select HAVE_PERF_REGS
select HAVE_PERF_USER_STACK_DUMP
+ select HAVE_PREEMPT_DYNAMIC_KEY
select HAVE_REGS_AND_STACK_ACCESS_API
select HAVE_POSIX_CPU_TIMERS_TASK_WORK
select HAVE_FUNCTION_ARG_ACCESS_API
diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
index e83f0982b99c..73623f719d7e 100644
--- a/arch/arm64/include/asm/preempt.h
+++ b/arch/arm64/include/asm/preempt.h
@@ -80,10 +80,22 @@ static inline bool should_resched(int preempt_offset)
}

#ifdef CONFIG_PREEMPTION
+#ifdef CONFIG_PREEMPT_DYNAMIC
+
+DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
+void dynamic_preempt_schedule(void);
+#define __preempt_schedule() dynamic_preempt_schedule()
+void dynamic_preempt_schedule_notrace(void);
+#define __preempt_schedule_notrace() dynamic_preempt_schedule_notrace()
+
+#else
+
void preempt_schedule(void);
-#define __preempt_schedule() preempt_schedule()
+#define __preempt_schedule() preempt_schedule()
void preempt_schedule_notrace(void);
-#define __preempt_schedule_notrace() preempt_schedule_notrace()
+#define __preempt_schedule_notrace() preempt_schedule_notrace()
+
+#endif
#endif /* CONFIG_PREEMPTION */

#endif /* __ASM_PREEMPT_H */
diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index f7408edf8571..f096678dd580 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -220,10 +220,19 @@ static void noinstr arm64_exit_el1_dbg(struct pt_regs *regs)
lockdep_hardirqs_on(CALLER_ADDR0);
}

+#ifdef CONFIG_PREEMPT_DYNAMIC
+DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
+#endif
+
static void __sched arm64_preempt_schedule_irq(void)
{
lockdep_assert_irqs_disabled();

+#ifdef CONFIG_PREEMPT_DYNAMIC
+ if (!static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
+ return;
+#endif
+
/*
* DAIF.DA are cleared at the start of IRQ/FIQ handling, and when GIC
* priority masking is used the GIC irqchip driver will clear DAIF.IF
--
2.11.0