linux-kernel - Re: [PATCH -next v5 04/22] arm64: entry: Rework arm64_preempt_schedule

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Message-ID: <Z6nj81GG_dLOzozI@J2N7QTR9R3>
Date: Mon, 10 Feb 2025 11:33:07 +0000
From: Mark Rutland <mark.rutland@....com>
To: Jinjie Ruan <ruanjinjie@...wei.com>
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Subject: Re: [PATCH -next v5 04/22] arm64: entry: Rework
 arm64_preempt_schedule_irq()

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@...wei.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
>