Re: [PATCH -next v5 04/22] arm64: entry: Rework arm64_preempt_schedule_irq()

Mark Rutland
2025-02-10T11:45:49Z

On Fri, Dec 06, 2024 at 06:17:26PM +0800, Jinjie Ruan wrote:
> The generic entry do preempt_schedule_irq() by checking if need_resched()
> satisfied, but arm64 has some of its own additional checks such as
> GIC priority masking.
>
> In preparation for moving arm64 over to the generic entry code, rework
> arm64_preempt_schedule_irq() to check whether it need resched in a check
> function called arm64_need_resched().

I think what this is saying is that the generic entry code has the form:

| raw_irqentry_exit_cond_resched()
| {
| if (!preempt_count()) {
| ...
| if (need_resched())
| preempt_schedule_irq();
| }
| }

... but it's not obvious why it's better to have and
arm64_need_resched() rather than a arm64_preempt_schedule_irq().

Having some idea of the change you intend to make to the generic code
would be helpful, and/or that generic change should be made earlier as a
preparatory patch.

Mark.

> No functional changes.
>
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> ---
> arch/arm64/kernel/entry-common.c | 17 ++++++++++-------
> 1 file changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 7a588515ee07..da68c089b74b 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -83,10 +83,10 @@ DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
> #define need_irq_preemption() (IS_ENABLED(CONFIG_PREEMPTION))
> #endif
>
> -static void __sched arm64_preempt_schedule_irq(void)
> +static inline bool arm64_need_resched(void)
> {
> if (!need_irq_preemption())
> - return;
> + return false;
>
> /*
> * Note: thread_info::preempt_count includes both thread_info::count
> @@ -94,7 +94,7 @@ static void __sched arm64_preempt_schedule_irq(void)
> * preempt_count().
> */
> if (READ_ONCE(current_thread_info()->preempt_count) != 0)
> - return;
> + return false;
>
> /*
> * DAIF.DA are cleared at the start of IRQ/FIQ handling, and when GIC
> @@ -103,7 +103,7 @@ static void __sched arm64_preempt_schedule_irq(void)
> * DAIF we must have handled an NMI, so skip preemption.
> */
> if (system_uses_irq_prio_masking() && read_sysreg(daif))
> - return;
> + return false;
>
> /*
> * Preempting a task from an IRQ means we leave copies of PSTATE
> @@ -113,8 +113,10 @@ static void __sched arm64_preempt_schedule_irq(void)
> * Only allow a task to be preempted once cpufeatures have been
> * enabled.
> */
> - if (system_capabilities_finalized())
> - preempt_schedule_irq();
> + if (!system_capabilities_finalized())
> + return false;
> +
> + return true;
> }
>
> /*
> @@ -139,7 +141,8 @@ static __always_inline void __exit_to_kernel_mode(struct pt_regs *regs,
> return;
> }
>
> - arm64_preempt_schedule_irq();
> + if (arm64_need_resched())
> + preempt_schedule_irq();
>
> trace_hardirqs_on();
> } else {
> --
> 2.34.1
>