genirq/proc: Fix missing CPU header in /proc/interrupts

commit e74798f2f8e1562f2bbd2fed8d81929eeef39ffc upstream.

irq_seq_show() must be idempotent since it may be called multiple times
to render the same record, e.g. when seq_file's buffer is exceeded. It
emits the CPU header behind the one-shot print_header flag, which the
first pass clears, so the header is lost with the discarded buffer.

This can be seen on large systems, such as 8g.48xl (192 CPUs) and
8i.96xl (384 CPUs) AWS instance types:

  $ head -1 /proc/interrupts
     9:          0          0 ...  0  GICv3  25 Level  vgic

Replace print_header with the position of the first record and include
the header whenever that record is emitted.

Fixes: 171cc0d9eed1 ("genirq/proc: Speed up /proc/interrupts iteration")
Signed-off-by: Haris Okanovic <harisokn@amazon.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Radu Rendec <radu@rendec.net>
Link: https://patch.msgid.link/20260911-dev-harisokn-proc-interrupts-fix-b4-v1-1-f9ff208efcfa@amazon.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/irq/proc.c

diff --git a/kernel/irq/proc.c b/kernel/irq/proc.c
index 1b83572..0c0df16 100644
--- a/kernel/irq/proc.c
+++ b/kernel/irq/proc.c
@@ -462,7 +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;
+	loff_t		first_pos;
 	unsigned int	num_prec;
 	unsigned int	chip_width;
 } irq_proc_constraints __read_mostly = {
@@ -543,14 +543,13 @@ static int irq_seq_show(struct seq_file *p, void *v)
 	struct irqaction *action;
 
 	/* Print header for the first interrupt? */
-	if (constr->print_header) {
+	if (p->index == constr->first_pos) {
 		unsigned int cpu;
 
 		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;
 	}
 
 	if (desc == ARCH_PROC_IRQDESC)
@@ -635,14 +634,25 @@ static void *irq_seq_next_desc(loff_t *pos)
 
 static void *irq_seq_start(struct seq_file *f, loff_t *pos)
 {
-	if (!*pos) {
-		struct irq_proc_constraints *constr = f->private;
+	struct irq_proc_constraints *constr = f->private;
+	bool first = !*pos;
+	void *ret;
 
+	if (first) {
 		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);
+
+	ret = irq_seq_next_desc(pos);
+
+	/*
+	 * Record position of the first output record so that irq_seq_show()
+	 * knows where to write the header.
+	 */
+	if (first)
+		constr->first_pos = *pos;
+
+	return ret;
 }
 
 static void *irq_seq_next(struct seq_file *f, void *v, loff_t *pos)