Re: [RFC PATCH v3 1/3] softirq: Add generic accessor to percpu softirq_pending data

[Heiko Carstens <hca@linux.ibm.com>]

On Wed, Sep 21, 2022 at 01:25:48AM +0000, John Stultz wrote:
> In a previous iteration of this patch series, I was checking:
> 
>    per_cpu(irq_stat, cpu).__softirq_pending
> 
> which resulted in build errors on s390.
> 
> This patch tries to create a generic accessor to this percpu
> softirq_pending data.
> 
> This interface is inherently racy as its reading percpu data
> without a lock. However, being able to peek at the softirq
> pending data allows us to make better decisions about rt task
> placement vs just ignoring it.
> 
> On s390 this call returns 0, which maybe isn't ideal but
> results in no functional change from what we do now.
> 
> Feedback or suggestions for better approach here would be
> welcome!
...
> diff --git a/arch/s390/include/asm/hardirq.h b/arch/s390/include/asm/hardirq.h
> index 58668ffb5488..cd9cc11588ab 100644
> --- a/arch/s390/include/asm/hardirq.h
> +++ b/arch/s390/include/asm/hardirq.h
> @@ -16,6 +16,12 @@
>  #define local_softirq_pending() (S390_lowcore.softirq_pending)
>  #define set_softirq_pending(x) (S390_lowcore.softirq_pending = (x))
>  #define or_softirq_pending(x)  (S390_lowcore.softirq_pending |= (x))
> +/*
> + *  Not sure what the right thing is here  for s390,
> + *  but returning 0 will result in no logical change
> + *  from what happens now
> + */
> +#define __cpu_softirq_pending(x) (0)

Hm, yes. This would require some ugly code to make this work properly,
plus might cause unique problems on s390 if the cpu in question is
offline.
So I'd say we might change s390 to use a proper per cpu irqstat
variable instead, if this new interface is being agreed on.