Re: [PATCH] loongson: Fix idle VS timer enqueue

[bibo mao]

yes, it had better skip out inner loop and let idle framework handle it.

Tested-by: Bibo Mao <maobibo@xxxxxxxxxxx>

On 2025/2/7 上午12:29, Marco Crivellari wrote:
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>
> Loongson re-enables interrupts on its idle routine and performs a
> TIF_NEED_RESCHED check afterwards before putting the CPU to sleep.
>
> The IRQs firing between the check and the idling instruction may set the
> TIF_NEED_RESCHED flag. In order to deal with the such a race, IRQs
> interrupting __arch_cpu_idle() rollback their return address to the
> beginning of __arch_cpu_idle() so that TIF_NEED_RESCHED is checked
> again before going back to sleep.
>
> However idle IRQs can also queue timers that may require a tick
> reprogramming through a new generic idle loop iteration but those timers
> would go unnoticed here because __arch_cpu_idle() only checks
> TIF_NEED_RESCHED. It doesn't check for pending timers.
>
> Fix this with fast-forwarding idle IRQs return value to the end of the
> idle routine instead of the beginning, so that the generic idle loop
> handles both TIF_NEED_RESCHED and pending timers.
>
> Fixes: 0603839b18f4 ("LoongArch: Add exception/interrupt handling")
> Not-yet-signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: WANG Xuerui <kernel@xxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Signed-off-by: Marco Crivellari <marco.crivellari@xxxxxxxx>
> ---
>   arch/loongarch/kernel/genex.S | 33 +++++++++++++++++++--------------
>   arch/loongarch/kernel/idle.c  |  3 +--
>   arch/loongarch/kernel/reset.c | 23 ++++++++++++++---------
>   3 files changed, 34 insertions(+), 25 deletions(-)
>
> diff --git a/arch/loongarch/kernel/genex.S b/arch/loongarch/kernel/genex.S
> index 86d5d90ebefe..ab2e9a262ee2 100644
> --- a/arch/loongarch/kernel/genex.S
> +++ b/arch/loongarch/kernel/genex.S
> @@ -18,28 +18,33 @@
>   
>   	.align	5
>   SYM_FUNC_START(__arch_cpu_idle)
> -	/* start of rollback region */
> -	LONG_L	t0, tp, TI_FLAGS
> -	nop
> -	andi	t0, t0, _TIF_NEED_RESCHED
> -	bnez	t0, 1f
> -	nop
> -	nop
> -	nop
> +	/* start of idle interrupt region */
> +	li.w	t0, CSR_CRMD_IE
> +	/* idle instruction needs irq enabled */
> +	csrxchg	t0, t0, LOONGARCH_CSR_CRMD
> +	/*
> +	 * If an interrupt lands here; between enabling interrupts above and
> +	 * going idle on the next instruction, we must *NOT* go idle since the
> +	 * interrupt could have set TIF_NEED_RESCHED or caused an timer to need
> +	 * reprogramming. Fall through -- see handle_vint() below -- and have
> +	 * the idle loop take care of things.
> +	 */
>   	idle	0
> -	/* end of rollback region */
> -1:	jr	ra
> +	nop
> +	/* end of idle interrupt region */
> +SYM_INNER_LABEL(__arch_cpu_idle_exit, SYM_L_LOCAL)
> +	jr	ra
>   SYM_FUNC_END(__arch_cpu_idle)
>   
>   SYM_CODE_START(handle_vint)
>   	UNWIND_HINT_UNDEFINED
>   	BACKUP_T0T1
>   	SAVE_ALL
> -	la_abs	t1, __arch_cpu_idle
> +	la_abs	t1, __arch_cpu_idle_exit
>   	LONG_L	t0, sp, PT_ERA
> -	/* 32 byte rollback region */
> -	ori	t0, t0, 0x1f
> -	xori	t0, t0, 0x1f
> +	/* 16 byte idle interrupt region */
> +	ori	t0, t0, 0x0f
> +	addi.d	t0, t0, 1
>   	bne	t0, t1, 1f
>   	LONG_S	t0, sp, PT_ERA
>   1:	move	a0, sp
> diff --git a/arch/loongarch/kernel/idle.c b/arch/loongarch/kernel/idle.c
> index 0b5dd2faeb90..54b247d8cdb6 100644
> --- a/arch/loongarch/kernel/idle.c
> +++ b/arch/loongarch/kernel/idle.c
> @@ -11,7 +11,6 @@
>   
>   void __cpuidle arch_cpu_idle(void)
>   {
> -	raw_local_irq_enable();
> -	__arch_cpu_idle(); /* idle instruction needs irq enabled */
> +	__arch_cpu_idle();
>   	raw_local_irq_disable();
>   }
> diff --git a/arch/loongarch/kernel/reset.c b/arch/loongarch/kernel/reset.c
> index 1ef8c6383535..d78e8a1064a1 100644
> --- a/arch/loongarch/kernel/reset.c
> +++ b/arch/loongarch/kernel/reset.c
> @@ -20,6 +20,11 @@
>   void (*pm_power_off)(void);
>   EXPORT_SYMBOL(pm_power_off);
>   
> +static __always_inline void native_halt(void)
> +{
> +	asm volatile("idle 0" : : : "memory");
> +}
> +
>   void machine_halt(void)
>   {
>   #ifdef CONFIG_SMP
> @@ -32,9 +37,9 @@ void machine_halt(void)
>   	pr_notice("\n\n** You can safely turn off the power now **\n\n");
>   	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
>   
> -	while (true) {
> -		__arch_cpu_idle();
> -	}
> +	while (1) {
> +		native_halt();
> +	};
>   }
>   
>   void machine_power_off(void)
> @@ -52,9 +57,9 @@ void machine_power_off(void)
>   	efi.reset_system(EFI_RESET_SHUTDOWN, EFI_SUCCESS, 0, NULL);
>   #endif
>   
> -	while (true) {
> -		__arch_cpu_idle();
> -	}
> +	while (1) {
> +		native_halt();
> +	};
>   }
>   
>   void machine_restart(char *command)
> @@ -73,7 +78,7 @@ void machine_restart(char *command)
>   	if (!acpi_disabled)
>   		acpi_reboot();
>   
> -	while (true) {
> -		__arch_cpu_idle();
> -	}
> +	while (1) {
> +		native_halt();
> +	};
>   }