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    Subject[PATCH 6/6] arm64: support PREEMPT_DYNAMIC
    Date
    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
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