[PATCH 3/3] powerpc/irq: Suppress unlikely interrupt stats by default

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Some interrupts are always zero and that is expected since they occur
very rarely and are mostly error indications. Don't print them by
default.

"MCE" -  "Machine check exceptions"
"NMI" -  "System Reset interrupts"

Print them if they occur once. Maintain a bitmap to know which
interrupts are to be printed.

Time taken to read /proc/interrupts 1000 times.
Base and v6 details can be found in cover-letter.
Base            :  103us
v6              :   63us
v6+patch 1+2    :   57us
v6+patch 1+2+3  :   54us

Patch 3 shows an additional 5% gain compared to patch 1+2. So it does
make sense to print them only if they are ever set.

Note: Since /proc/interrupts depend on kconfig and arch dependent,
userspace tools don't make explicit assumptions.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 arch/powerpc/include/asm/hardirq.h |  1 +
 arch/powerpc/kernel/irq.c          | 37 +++++++++++++++++++++++++++---
 arch/powerpc/kernel/traps.c        |  4 ++--
 3 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/hardirq.h b/arch/powerpc/include/asm/hardirq.h
index 38098e35b241..be6cd5aab016 100644
--- a/arch/powerpc/include/asm/hardirq.h
+++ b/arch/powerpc/include/asm/hardirq.h
@@ -31,6 +31,7 @@ DECLARE_PER_CPU(unsigned int, __softirq_pending);
 #define local_softirq_pending_ref       __softirq_pending
 
 #define inc_irq_stat(index)	__this_cpu_inc(irq_stat.counts[IRQ_COUNT_##index])
+void inc_irq_stat_and_enable(enum irq_stat_counts which);
 
 #define __ARCH_IRQ_STAT
 #define __ARCH_IRQ_EXIT_IRQS_DISABLED
diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index e67a18f62142..048ddfa66fc4 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -87,9 +87,13 @@ u32 tau_interrupts(unsigned long cpu);
 struct irq_stat_info {
 	const char	*symbol;
 	const char	*text;
+	int		skip;
 };
 
-#define ISE(idx, sym, txt)[IRQ_COUNT_##idx] = { .symbol = sym, .text = txt}
+/* ISE - IRQ STAT ENABLED, ISC - IRQ STAT CONDITIONAL */
+#define ISE(idx, sym, txt)[IRQ_COUNT_##idx] = { .symbol = sym, .text = txt, .skip = 0}
+#define ISC(idx, sym, txt)[IRQ_COUNT_##idx] = { .symbol = sym, .text = txt, .skip = 1}
+
 
 static struct irq_stat_info irq_stat_info[IRQ_COUNT_MAX] __ro_after_init = {
 	ISE(LOC_TIMER,		"LOC", "  Local timer interrupts for timer event device\n"),
@@ -97,8 +101,8 @@ static struct irq_stat_info irq_stat_info[IRQ_COUNT_MAX] __ro_after_init = {
 	ISE(OTHER_TIMER,	"LOC", "  Local timer interrupts for others\n"),
 	ISE(SPURIOUS,		"SPU", "  Spurious interrupts\n"),
 	ISE(PMI,		"PMI", "  Performance monitoring interrupts\n"),
-	ISE(MCE,		"MCE", "  Machine check exceptions\n"),
-	ISE(NMI_SRESET,		"NMI", "  System Reset interrupts\n"),
+	ISC(MCE,		"MCE", "  Machine check exceptions\n"),
+	ISC(NMI_SRESET,		"NMI", "  System Reset interrupts\n"),
 #ifdef CONFIG_PPC_WATCHDOG
 	ISE(WATCHDOG,		"WDG", "  Watchdog soft-NMI interrupts\n"),
 #endif
@@ -107,11 +111,25 @@ static struct irq_stat_info irq_stat_info[IRQ_COUNT_MAX] __ro_after_init = {
 #endif
 };
 
+/*
+ * Used for default disabled counters to increment the stats and to enable the
+ * entry for /proc/interrupts output.
+ */
+static DECLARE_BITMAP(irq_stat_count_show, IRQ_COUNT_MAX) __read_mostly;
+void inc_irq_stat_and_enable(enum irq_stat_counts which)
+{
+	__this_cpu_inc(irq_stat.counts[which]);
+	set_bit(which, irq_stat_count_show);
+}
+
 int arch_show_interrupts(struct seq_file *p, int prec)
 {
 	const struct irq_stat_info *info = irq_stat_info;
 
 	for (unsigned int i = 0; i < ARRAY_SIZE(irq_stat_info); i++, info++) {
+		if (!test_bit(i, irq_stat_count_show))
+			continue;
+
 		seq_printf(p, "%*s:", prec, info->symbol);
 		irq_proc_emit_counts(p, &irq_stat.counts[i]);
 		seq_puts(p, info->text);
@@ -138,6 +156,19 @@ int arch_show_interrupts(struct seq_file *p, int prec)
 	return 0;
 }
 
+static int __init irq_init_stats(void)
+{
+	struct irq_stat_info *info = irq_stat_info;
+
+	for (unsigned int i = 0; i < ARRAY_SIZE(irq_stat_info); i++, info++) {
+		if (info->skip == 0)
+			set_bit(i, irq_stat_count_show);
+	}
+
+	return 0;
+}
+late_initcall(irq_init_stats);
+
 /*
  * /proc/stat helpers
  */
diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index a8f15154bd9a..3eacbd20fc80 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -459,7 +459,7 @@ DEFINE_INTERRUPT_HANDLER_NMI(system_reset_exception)
 	}
 
 	hv_nmi_check_nonrecoverable(regs);
-	inc_irq_stat(NMI_SRESET);
+	inc_irq_stat_and_enable(IRQ_COUNT_NMI_SRESET);
 
 	/* See if any machine dependent calls */
 	if (ppc_md.system_reset_exception) {
@@ -816,7 +816,7 @@ static void __machine_check_exception(struct pt_regs *regs)
 {
 	int recover = 0;
 
-	inc_irq_stat(MCE);
+	inc_irq_stat_and_enable(IRQ_COUNT_MCE);
 
 	add_taint(TAINT_MACHINE_CHECK, LOCKDEP_NOW_UNRELIABLE);
 
-- 
2.47.3