Re: [PATCH -next v5 03/22] arm64: entry: Move arm64_preempt_schedule_irq() into __exit_to_kernel_mode()

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Dec 06, 2024 at 06:17:25PM +0800, Jinjie Ruan wrote:
> The generic entry code try to reschedule every time when the kernel
> mode non-NMI exception return. At the moment, arm64 only reschedule every
> time when EL1 irq exception return;

I think this is a bit unclear, and should say something like:
  
| The arm64 entry code only preempts a kernel context upon a return from
| a regular IRQ exception. The generic entry code may preempt a kernel
| context for any exception return where irqentry_exit() is used, and so
| may preempt other exceptions such as faults.

Thomas, can you confirm that's the *intent* of the generic entry code?

> In preparation for moving arm64 over to the generic entry code, move
> arm64_preempt_schedule_irq() into exit_to_kernel_mode(), so not
> only EL1 irq but also all EL1 non-NMI exception return, there is a chance
> to reschedule. And only if irqs are enabled when the exception trapped,
> there may be a chance to reschedule after the exceptions have been handled,
> so move arm64_preempt_schedule_irq() into regs_irqs_disabled()
> check false block, but it will try to reschedule only when TINY_RCU is
> enabled or current is not an idle task.

I think the detail is confusing here, and it would be better to say:

| In preparation for moving arm64 over to the generic entry code, align
| arm64 with the generic behaviour by calling
| arm64_preempt_schedule_irq() from exit_to_kernel_mode(). To make this
| possible, arm64_preempt_schedule_irq() and need_irq_preemption() are
| moved earlier in the file, with no changes.

Mark.

> As Mark pointed out, this change will have the following 2 key impact:
> 
> - " We'll preempt even without taking a "real" interrupt. That
>     shouldn't result in preemption that wasn't possible before,
>     but it does change the probability of preempting at certain points,
>     and might have a performance impact, so probably warrants a
>     benchmark."
> 
> - " We will not preempt when taking interrupts from a region of kernel
>     code where IRQs are enabled but RCU is not watching, matching the
>     behaviour of the generic entry code.
> 
>     This has the potential to introduce livelock if we can ever have a
>     screaming interrupt in such a region, so we'll need to go figure out
>     whether that's actually a problem.
> 
>     Having this as a separate patch will make it easier to test/bisect
>     for that specifically."
> 
> Suggested-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> ---
>  arch/arm64/kernel/entry-common.c | 88 ++++++++++++++++----------------
>  1 file changed, 44 insertions(+), 44 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 1687627b2ecf..7a588515ee07 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -75,6 +75,48 @@ static noinstr irqentry_state_t enter_from_kernel_mode(struct pt_regs *regs)
>  	return state;
>  }
>  
> +#ifdef CONFIG_PREEMPT_DYNAMIC
> +DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
> +#define need_irq_preemption() \
> +	(static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
> +#else
> +#define need_irq_preemption()	(IS_ENABLED(CONFIG_PREEMPTION))
> +#endif
> +
> +static void __sched arm64_preempt_schedule_irq(void)
> +{
> +	if (!need_irq_preemption())
> +		return;
> +
> +	/*
> +	 * Note: thread_info::preempt_count includes both thread_info::count
> +	 * and thread_info::need_resched, and is not equivalent to
> +	 * preempt_count().
> +	 */
> +	if (READ_ONCE(current_thread_info()->preempt_count) != 0)
> +		return;
> +
> +	/*
> +	 * 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
> +	 * using gic_arch_enable_irqs() for normal IRQs. If anything is set in
> +	 * DAIF we must have handled an NMI, so skip preemption.
> +	 */
> +	if (system_uses_irq_prio_masking() && read_sysreg(daif))
> +		return;
> +
> +	/*
> +	 * Preempting a task from an IRQ means we leave copies of PSTATE
> +	 * on the stack. cpufeature's enable calls may modify PSTATE, but
> +	 * resuming one of these preempted tasks would undo those changes.
> +	 *
> +	 * Only allow a task to be preempted once cpufeatures have been
> +	 * enabled.
> +	 */
> +	if (system_capabilities_finalized())
> +		preempt_schedule_irq();
> +}
> +
>  /*
>   * Handle IRQ/context state management when exiting to kernel mode.
>   * After this function returns it is not safe to call regular kernel code,
> @@ -97,6 +139,8 @@ static __always_inline void __exit_to_kernel_mode(struct pt_regs *regs,
>  			return;
>  		}
>  
> +		arm64_preempt_schedule_irq();
> +
>  		trace_hardirqs_on();
>  	} else {
>  		if (state.exit_rcu)
> @@ -281,48 +325,6 @@ 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);
> -#define need_irq_preemption() \
> -	(static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
> -#else
> -#define need_irq_preemption()	(IS_ENABLED(CONFIG_PREEMPTION))
> -#endif
> -
> -static void __sched arm64_preempt_schedule_irq(void)
> -{
> -	if (!need_irq_preemption())
> -		return;
> -
> -	/*
> -	 * Note: thread_info::preempt_count includes both thread_info::count
> -	 * and thread_info::need_resched, and is not equivalent to
> -	 * preempt_count().
> -	 */
> -	if (READ_ONCE(current_thread_info()->preempt_count) != 0)
> -		return;
> -
> -	/*
> -	 * 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
> -	 * using gic_arch_enable_irqs() for normal IRQs. If anything is set in
> -	 * DAIF we must have handled an NMI, so skip preemption.
> -	 */
> -	if (system_uses_irq_prio_masking() && read_sysreg(daif))
> -		return;
> -
> -	/*
> -	 * Preempting a task from an IRQ means we leave copies of PSTATE
> -	 * on the stack. cpufeature's enable calls may modify PSTATE, but
> -	 * resuming one of these preempted tasks would undo those changes.
> -	 *
> -	 * Only allow a task to be preempted once cpufeatures have been
> -	 * enabled.
> -	 */
> -	if (system_capabilities_finalized())
> -		preempt_schedule_irq();
> -}
> -
>  static void do_interrupt_handler(struct pt_regs *regs,
>  				 void (*handler)(struct pt_regs *))
>  {
> @@ -591,8 +593,6 @@ static __always_inline void __el1_irq(struct pt_regs *regs,
>  	do_interrupt_handler(regs, handler);
>  	irq_exit_rcu();
>  
> -	arm64_preempt_schedule_irq();
> -
>  	exit_to_kernel_mode(regs, state);
>  }
>  static void noinstr el1_interrupt(struct pt_regs *regs,
> -- 
> 2.34.1
>