Re: [patch V4 15/15] genirq/proc: Speed up /proc/interrupts iteration

[Dmitry Ilvokhin <d@ilvokhin.com>]

On Tue, Mar 31, 2026 at 09:26:40AM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@kernel.org>
> 
> 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>
> ---

[...]

>  
> -	/* 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);
> +	/* print header for the first interrupt indicated by !p>private */

nit: This comment seems a bit out of place here, since we are looking at
constr->print_header directly here.

> +	if (constr->print_header) {
> +		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;
>  	}

While at it, moving the print_header block above the ARCH_PROC_IRQDESC
check would make the code more robust. If there are no valid descriptors
to display, the CPU column header would still be printed before
arch_show_interrupts() output. Not a practical issue, but it makes the
logic obviously correct.

Reviewed-by: Dmitry Ilvokhin <d@ilvokhin.com>