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!
Cc: John Dias <[email protected]>
Cc: Connor O'Brien <[email protected]>
Cc: Rick Yiu <[email protected]>
Cc: John Kacur <[email protected]>
Cc: Qais Yousef <[email protected]>
Cc: Chris Redpath <[email protected]>
Cc: Abhijeet Dharmapurikar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Vincent Guittot <[email protected]>
Cc: Dietmar Eggemann <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: [email protected]
Reported-by: kernel test robot <[email protected]>
Signed-off-by: John Stultz <[email protected]>
---
arch/s390/include/asm/hardirq.h | 6 ++++++
include/linux/interrupt.h | 11 +++++++++++
2 files changed, 17 insertions(+)
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)
#define __ARCH_IRQ_STAT
#define __ARCH_IRQ_EXIT_IRQS_DISABLED
diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index a92bce40b04b..a749a8663841 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -527,6 +527,17 @@ DECLARE_STATIC_KEY_FALSE(force_irqthreads_key);
#define set_softirq_pending(x) (__this_cpu_write(local_softirq_pending_ref, (x)))
#define or_softirq_pending(x) (__this_cpu_or(local_softirq_pending_ref, (x)))
+/**
+ * __cpu_softirq_pending() - Checks to see if softirq is pending on a cpu
+ *
+ * This helper is inherently racy, as we're accessing per-cpu data w/o locks.
+ * But peeking at the flag can still be useful when deciding where to place a
+ * task.
+ */
+static inline u32 __cpu_softirq_pending(int cpu)
+{
+ return (u32)per_cpu(local_softirq_pending_ref, cpu);
+}
#endif /* local_softirq_pending */
/* Some architectures might implement lazy enabling/disabling of
--
2.37.3.968.ga6b4b080e4-goog
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.