Re: [PATCH] genirq/proc: Fix missing CPU header in /proc/interrupts

[Radu Rendec]

On Fri, 2026-09-11 at 13:31 -0500, Haris Okanovic wrote:
> 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@xxxxxxxxxx>
> ---
>  kernel/irq/proc.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/irq/proc.c b/kernel/irq/proc.c
> index 1b835725f7b1c..0c0df16de5816 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)
> 
> ---
> base-commit: 08df884136f1c1197bab2a27814404fd329d9aac
> change-id: 20260911-dev-harisokn-proc-interrupts-fix-b4-7498f7e18db5

Since I don't have access to a machine with so many CPUs, I tested
locally in a regular VM and simulated the large number of CPUs by
adding artificial buffer writes to both the header part and the IRQ
counter part of irq_seq_show(). I could reproduce the problem and
validate the fix. Also, semantically the fix looks good to me.
Thanks for catching this and for the patch!

Reviewed-by: Radu Rendec <radu@xxxxxxxxxx>