Subject: Re: [PATCHv11 2/4] genirq: Provide a snapshot mechanism for interrupt statistics

From: Thomas Gleixner <[email-masked]>
Date: Fri, 01 Mar 2024 20:22:54 +0100

Doug!

On Wed, Feb 28 2024 at 14:44, Doug Anderson wrote:
> I won't insist on it, but I continue to worry about memory
> implications with large numbers of CPUs. With a 4-byte int, 8192 max
> CPUs, and 100 IRQs the extra "ref" value takes up over 3MB of memory
> (8192 * 4 bytes * 100).
>
> Technically, you could add a new symbol like "config
> NEED_IRQ_SNAPSHOTS". This wouldn't be a symbol selectable by the end
> user but would automatically be selected by "config
> SOFTLOCKUP_DETECTOR_INTR_STORM". If the config wasn't defined then the
> struct wouldn't contain "ref" and the snapshot routines would just be
> static inline stubs.
>
> Maybe Thomas has an opinion about whether this is something to worry
> about. Worst case it wouldn't be hard to do in a follow-up patch.

I'd say it makes sense to give people a choice to save memory especially
when the softlock detector code is not enabled.

It's rather straight forward to do.

Thanks,

        tglx
---
--- a/include/linux/irqdesc.h
+++ b/include/linux/irqdesc.h
@@ -24,7 +24,9 @@ struct pt_regs;
  */
 struct irqstat {
 	unsigned int	cnt;
+#ifdef CONFIG_GENIRQ_STAT_SNAPSHOT
 	unsigned int	ref;
+#endif
 };
 
 /**
--- a/kernel/irq/irqdesc.c
+++ b/kernel/irq/irqdesc.c
@@ -978,6 +978,7 @@ static unsigned int kstat_irqs(unsigned
 	return sum;
 }
 
+#ifdef CONFIG_GENIRQ_STAT_SNAPSHOT
 void kstat_snapshot_irqs(void)
 {
 	struct irq_desc *desc;
@@ -998,6 +999,7 @@ unsigned int kstat_get_irq_since_snapsho
 		return 0;
 	return this_cpu_read(desc->kstat_irqs->cnt) - this_cpu_read(desc->kstat_irqs->ref);
 }
+#endif
 
 /**
  * kstat_irqs_usr - Get the statistics for an interrupt from thread context
--- a/kernel/irq/Kconfig
+++ b/kernel/irq/Kconfig
@@ -108,6 +108,10 @@ config GENERIC_IRQ_MATRIX_ALLOCATOR
 config GENERIC_IRQ_RESERVATION_MODE
 	bool
 
+# Snapshot for interrupt statistics
+config GENERIC_IRQ_STAT_SNAPSHOT
+	bool
+
 # Support forced irq threading
 config IRQ_FORCED_THREADING
        bool