Subject: Re: [RFC] powerpc: handle simultanneous interrupts at once

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 14 Mar 2017 10:42:27 +1100

On Fri, 2017-03-10 at 12:11 +0100, Christophe Leroy wrote:
> It often happens to have simultanneous interrupts, for instance
> when having double Ethernet attachment. With the current
> implementation, we suffer the cost of kernel entry/exit for each
> interrupt.
> 
> This patch introduces a loop in __do_irq() to handle all interrupts
> at once before returning.
> 
> Signed-off-by: Christophe Leroy <[email-masked]>
> ---
>  arch/powerpc/include/asm/hw_irq.h |  6 ++++++
>  arch/powerpc/kernel/irq.c         | 22 +++++++++++++++-------
>  2 files changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/hw_irq.h
> b/arch/powerpc/include/asm/hw_irq.h
> index eba60416536e..d69ae5846955 100644
> --- a/arch/powerpc/include/asm/hw_irq.h
> +++ b/arch/powerpc/include/asm/hw_irq.h
> @@ -123,6 +123,11 @@ static inline void may_hard_irq_enable(void)
>  		__hard_irq_enable();
>  }
>  
> +static inline void may_hard_irq_disable(void)
> +{
> +	__hard_irq_disable();
> +}
> +
>  static inline bool arch_irq_disabled_regs(struct pt_regs *regs)
>  {
>  	return !regs->softe;
> @@ -204,6 +209,7 @@ static inline bool arch_irq_disabled_regs(struct
> pt_regs *regs)
>  }
>  
>  static inline void may_hard_irq_enable(void) { }
> +static inline void may_hard_irq_disable(void) { }
>  
>  #endif /* CONFIG_PPC64 */
>  
> diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
> index a018f5cae899..28aca510c166 100644
> --- a/arch/powerpc/kernel/irq.c
> +++ b/arch/powerpc/kernel/irq.c
> @@ -515,14 +515,22 @@ void __do_irq(struct pt_regs *regs)
>  	 */
>  	irq = ppc_md.get_irq();
>  
> -	/* We can hard enable interrupts now to allow perf
> interrupts */
> -	may_hard_irq_enable();
> +	do {
> +		/* We can hard enable interrupts now to allow perf
> interrupts */
> +		may_hard_irq_enable();
> +
> +		/* And finally process it */
> +		if (unlikely(!irq))
> +			__this_cpu_inc(irq_stat.spurious_irqs);
> +		else
> +			generic_handle_irq(irq);
> +
> +		may_hard_irq_disable();

Not sure the above is that useful. If another interrupt is pending,
on ppc64 at least, you will have got it right at may_hard_irq_enable()
which would have caused a hard-disable for you anyway.

> -	/* And finally process it */
> -	if (unlikely(!irq))
> -		__this_cpu_inc(irq_stat.spurious_irqs);
> -	else
> -		generic_handle_irq(irq);
> +		irq = ppc_md.get_irq();
> +	} while (irq);
> +
> +	may_hard_irq_enable();
>  
>  	trace_irq_exit(regs);
>