genirq/proc: Speed up /proc/interrupts iteration

Reading /proc/interrupts iterates over the interrupt number space one by
one and looks up the descriptors one by one. That's just a waste of time.

When CONFIG_GENERIC_IRQ_SHOW is enabled this can utilize the maple tree and
cache the descriptor pointer efficiently for the sequence file operations.

Implement a CONFIG_GENERIC_IRQ_SHOW specific version in the core code and
leave the fs/proc/ variant for the legacy architectures which ignore generic
code.

This reduces the time wasted for looking up the next record significantly.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Dmitry Ilvokhin <d@ilvokhin.com>
Link: https://patch.msgid.link/20260517194932.165280601@kernel.org

fs/proc/Makefile

@@ -16,7 +16,9 @@ proc-y	+= cmdline.o
 proc-y	+= consoles.o
 proc-y	+= cpuinfo.o
 proc-y	+= devices.o
-proc-y	+= interrupts.o
+ifneq ($(CONFIG_GENERIC_IRQ_SHOW),y)
+proc-y += interrupts.o
+endif
 proc-y	+= loadavg.o
 proc-y	+= meminfo.o
 proc-y	+= stat.o

kernel/irq/proc.c

@@ -452,6 +452,8 @@ void irq_proc_update_valid(struct irq_desc *desc)
 
 #ifdef CONFIG_GENERIC_IRQ_SHOW
 
+#define ARCH_PROC_IRQDESC ((void *)0x00001111)
+
 int __weak arch_show_interrupts(struct seq_file *p, int prec)
 {
 	return 0;
@@ -460,6 +462,7 @@ int __weak arch_show_interrupts(struct seq_file *p, int prec)
 static DEFINE_RAW_SPINLOCK(irq_proc_constraints_lock);
 
 static struct irq_proc_constraints {
+	bool		print_header;
 	unsigned int	num_prec;
 	unsigned int	chip_width;
 } irq_proc_constraints __read_mostly = {
@@ -533,34 +536,28 @@ void irq_proc_emit_counts(struct seq_file *p, unsigned int __percpu *cnts)
 	irq_proc_emit_zero_counts(p, zeros);
 }
 
-int show_interrupts(struct seq_file *p, void *v)
+static int irq_seq_show(struct seq_file *p, void *v)
 {
-	unsigned int chip_width = READ_ONCE(irq_proc_constraints.chip_width);
-	unsigned int prec = READ_ONCE(irq_proc_constraints.num_prec);
-	int i = *(loff_t *) v, j;
+	struct irq_proc_constraints *constr = p->private;
+	struct irq_desc *desc = v;
 	struct irqaction *action;
-	struct irq_desc *desc;
 
-	if (i > ACTUAL_NR_IRQS)
-		return 0;
+	/* Print header for the first interrupt? */
+	if (constr->print_header) {
+		unsigned int cpu;
 
-	if (i == ACTUAL_NR_IRQS)
-		return arch_show_interrupts(p, prec);
-
-	/* print header and calculate the width of the first column */
-	if (i == 0) {
-		seq_printf(p, "%*s", prec + 8, "");
-		for_each_online_cpu(j)
-			seq_printf(p, "CPU%-8d", j);
+		seq_printf(p, "%*s", constr->num_prec + 8, "");
+		for_each_online_cpu(cpu)
+			seq_printf(p, "CPU%-8d", cpu);
 		seq_putc(p, '\n');
+		constr->print_header = false;
 	}
 
-	guard(rcu)();
-	desc = irq_to_desc(i);
-	if (!desc || !irq_settings_proc_valid(desc))
-		return 0;
+	if (desc == ARCH_PROC_IRQDESC)
+		return arch_show_interrupts(p, constr->num_prec);
 
-	seq_printf(p, "%*d:", prec, i);
+	seq_put_decimal_ull_width(p, "", irq_desc_get_irq(desc), constr->num_prec);
+	seq_putc(p, ':');
 
 	/*
 	 * Always output per CPU interrupts. Output device interrupts only when
@@ -580,18 +577,18 @@ int show_interrupts(struct seq_file *p, void *v)
 		if (desc->irq_data.chip->irq_print_chip)
 			desc->irq_data.chip->irq_print_chip(&desc->irq_data, p);
 		else if (desc->irq_data.chip->name)
-			seq_printf(p, "%-*s", chip_width, desc->irq_data.chip->name);
+			seq_printf(p, "%-*s", constr->chip_width, desc->irq_data.chip->name);
 		else
-			seq_printf(p, "%-*s", chip_width, "-");
+			seq_printf(p, "%-*s", constr->chip_width, "-");
 	} else {
-		seq_printf(p, "%-*s", chip_width, "None");
+		seq_printf(p, "%-*s", constr->chip_width, "None");
 	}
 
 	seq_putc(p, ' ');
 	if (desc->irq_data.domain)
-		seq_put_decimal_ull_width(p, "", desc->irq_data.hwirq, prec);
+		seq_put_decimal_ull_width(p, "", desc->irq_data.hwirq, constr->num_prec);
 	else
-		seq_printf(p, " %*s", prec, "");
+		seq_printf(p, " %*s", constr->num_prec, "");
 
 	if (IS_ENABLED(CONFIG_GENERIC_IRQ_SHOW_LEVEL))
 		seq_printf(p, " %-8s", irqd_is_level_type(&desc->irq_data) ? "Level" : "Edge");
@@ -609,4 +606,73 @@ int show_interrupts(struct seq_file *p, void *v)
 	seq_putc(p, '\n');
 	return 0;
 }
+
+static void *irq_seq_next_desc(loff_t *pos)
+{
+	if (*pos > total_nr_irqs)
+		return NULL;
+
+	guard(rcu)();
+	for (;;) {
+		struct irq_desc *desc = irq_find_desc_at_or_after((unsigned int) *pos);
+
+		if (desc) {
+			*pos = irq_desc_get_irq(desc);
+			/*
+			 * If valid for output then try to acquire a reference
+			 * count on the descriptor so that it can't be freed
+			 * after dropping RCU read lock on return.
+			 */
+			if (irq_settings_proc_valid(desc) && irq_desc_get_ref(desc))
+				return desc;
+			(*pos)++;
+		} else {
+			*pos = total_nr_irqs;
+			return ARCH_PROC_IRQDESC;
+		}
+	}
+}
+
+static void *irq_seq_start(struct seq_file *f, loff_t *pos)
+{
+	if (!*pos) {
+		struct irq_proc_constraints *constr = f->private;
+
+		constr->num_prec = READ_ONCE(irq_proc_constraints.num_prec);
+		constr->chip_width = READ_ONCE(irq_proc_constraints.chip_width);
+		constr->print_header = true;
+	}
+	return irq_seq_next_desc(pos);
+}
+
+static void *irq_seq_next(struct seq_file *f, void *v, loff_t *pos)
+{
+	if (v && v != ARCH_PROC_IRQDESC)
+		irq_desc_put_ref(v);
+
+	(*pos)++;
+	return irq_seq_next_desc(pos);
+}
+
+static void irq_seq_stop(struct seq_file *f, void *v)
+{
+	if (v && v != ARCH_PROC_IRQDESC)
+		irq_desc_put_ref(v);
+}
+
+static const struct seq_operations irq_seq_ops = {
+	.start = irq_seq_start,
+	.next  = irq_seq_next,
+	.stop  = irq_seq_stop,
+	.show  = irq_seq_show,
+};
+
+static int __init irq_proc_init(void)
+{
+	proc_create_seq_private("interrupts", 0, NULL, &irq_seq_ops,
+				sizeof(irq_proc_constraints), NULL);
+	return 0;
+}
+fs_initcall(irq_proc_init);
+
 #endif