Re: [patch V5 05/15] x86/irq: Suppress unlikely interrupt stats by default

[Radu Rendec <radu@rendec.net>]

It looks like we had a "race condition", and you haven't picked up my
Reviewed-by tag on this patch on v4. There was a minor comment too. I'm
re-including them here.

On Wed, 2026-04-01 at 23:51 +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@kernel.org>
> 
> Unlikely interrupt counters like the spurious vector and the synthetic APIC
> ICR read retry show up in /proc/interrupts with all counts 0 most of the
> time.
> 
> As these are events which should never happen, suppress them by default and
> enable them for output when they actually happen.
> 
> This requires a seperate bitmap as the description array is marked
> __ro_after_init. With that bitmap in place it becomes RO data.
> 
> Signed-off-by: Thomas Gleixner <tglx@kernel.org>
> ---
> V5: Move irq_stat_inc_and_enable() here
> V4: Fix the bad idea of writing to __ro_after_init marked data
> V3: New patch
> ---
>  arch/x86/include/asm/hardirq.h |    1 +
>  arch/x86/kernel/apic/apic.c    |    2 +-
>  arch/x86/kernel/apic/ipi.c     |    2 +-
>  arch/x86/kernel/irq.c          |   38 ++++++++++++++++++++++++++++----------
>  4 files changed, 31 insertions(+), 12 deletions(-)
> --- a/arch/x86/include/asm/hardirq.h
> +++ b/arch/x86/include/asm/hardirq.h
> @@ -68,6 +68,7 @@ DECLARE_PER_CPU_ALIGNED(struct pi_desc,
>  #define __ARCH_IRQ_STAT
>  
>  #define inc_irq_stat(index)	this_cpu_inc(irq_stat.counts[IRQ_COUNT_##index])
> +void irq_stat_inc_and_enable(enum irq_stat_counts which);
>  
>  #ifdef CONFIG_X86_LOCAL_APIC
>  #define inc_perf_irq_stat()	inc_irq_stat(APIC_PERF)
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -2108,7 +2108,7 @@ static noinline void handle_spurious_int
>  
>  	trace_spurious_apic_entry(vector);
>  
> -	inc_irq_stat(SPURIOUS);
> +	irq_stat_inc_and_enable(IRQ_COUNT_SPURIOUS);

This is just a matter of style but for symmetry with inc_irq_stat(), I
would prepend '__' to the function name and create a wrapper macro that
adds the 'IRQ_COUNT_' prefix.

And for consistency (you also have inc_perf_irq_stat()) I would call it
inc_and_enable_irq_stat().

And yes, my comments above are totally bike shedding :)

>  
>  	/*
>  	 * If this is a spurious interrupt then do not acknowledge
> --- a/arch/x86/kernel/apic/ipi.c
> +++ b/arch/x86/kernel/apic/ipi.c
> @@ -120,7 +120,7 @@ u32 apic_mem_wait_icr_idle_timeout(void)
>  	for (cnt = 0; cnt < 1000; cnt++) {
>  		if (!(apic_read(APIC_ICR) & APIC_ICR_BUSY))
>  			return 0;
> -		inc_irq_stat(ICR_READ_RETRY);
> +		irq_stat_inc_and_enable(IRQ_COUNT_ICR_READ_RETRY);
>  		udelay(100);
>  	}
>  	return APIC_ICR_BUSY;
> --- a/arch/x86/kernel/irq.c
> +++ b/arch/x86/kernel/irq.c
> @@ -69,19 +69,24 @@ struct irq_stat_info {
>  	const char	*text;
>  };
>  
> +#define DEFAULT_SUPPRESSED_VECTOR	UINT_MAX
> +
>  #define ISS(idx, sym, txt) [IRQ_COUNT_##idx] = { .symbol = sym, .text = txt }
>  
>  #define ITS(idx, sym, txt) [IRQ_COUNT_##idx] =				\
>  	{ .skip_vector = idx## _VECTOR, .symbol = sym, .text = txt }
>  
> -static struct irq_stat_info irq_stat_info[IRQ_COUNT_MAX] __ro_after_init = {
> +#define IDS(idx, sym, txt) [IRQ_COUNT_##idx] =				\
> +	{ .skip_vector = DEFAULT_SUPPRESSED_VECTOR, .symbol = sym, .text = txt }
> +
> +static const struct irq_stat_info irq_stat_info[IRQ_COUNT_MAX] = {
>  	ISS(NMI,			"NMI", "  Non-maskable interrupts\n"),
>  #ifdef CONFIG_X86_LOCAL_APIC
>  	ISS(APIC_TIMER,			"LOC", "  Local timer interrupts\n"),
> -	ISS(SPURIOUS,			"SPU", "  Spurious interrupts\n"),
> +	IDS(SPURIOUS,			"SPU", "  Spurious interrupts\n"),
>  	ISS(APIC_PERF,			"PMI", "  Performance monitoring interrupts\n"),
>  	ISS(IRQ_WORK,			"IWI", "  IRQ work interrupts\n"),
> -	ISS(ICR_READ_RETRY,		"RTR", "  APIC ICR read retries\n"),
> +	IDS(ICR_READ_RETRY,		"RTR", "  APIC ICR read retries\n"),
>  	ISS(X86_PLATFORM_IPI,		"PLT", "  Platform interrupts\n"),
>  #endif
>  #ifdef CONFIG_SMP
> @@ -122,34 +127,47 @@ static struct irq_stat_info irq_stat_inf
>  #endif
>  };
>  
> +static DECLARE_BITMAP(irq_stat_count_show, IRQ_COUNT_MAX) __read_mostly;
> +
>  static int __init irq_init_stats(void)
>  {
> -	struct irq_stat_info *info = irq_stat_info;
> +	const struct irq_stat_info *info = irq_stat_info;
>  
>  	for (unsigned int i = 0; i < ARRAY_SIZE(irq_stat_info); i++, info++) {
> -		if (info->skip_vector && test_bit(info->skip_vector, system_vectors))
> -			info->skip_vector = 0;
> +		if (!info->skip_vector || (info->skip_vector != DEFAULT_SUPPRESSED_VECTOR &&
> +					   test_bit(info->skip_vector, system_vectors)))
> +			set_bit(i, irq_stat_count_show);
>  	}
>  
>  #ifdef CONFIG_X86_LOCAL_APIC
>  	if (!x86_platform_ipi_callback)
> -		irq_stat_info[IRQ_COUNT_X86_PLATFORM_IPI].skip_vector = 1;
> +		clear_bit(IRQ_COUNT_X86_PLATFORM_IPI, irq_stat_count_show);
>  #endif
>  
>  #ifdef CONFIG_X86_POSTED_MSI
>  	if (!posted_msi_enabled())
> -		irq_stat_info[IRQ_COUNT_POSTED_MSI_NOTIFICATION].skip_vector = 1;
> +		clear_bit(IRQ_COUNT_POSTED_MSI_NOTIFICATION, irq_stat_count_show);
>  #endif
>  
>  #ifdef CONFIG_X86_MCE_AMD
>  	if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
>  	    boot_cpu_data.x86_vendor != X86_VENDOR_HYGON)
> -		irq_stat_info[IRQ_COUNT_DEFERRED_ERROR].skip_vector = 1;
> +		clear_bit(IRQ_COUNT_DEFERRED_ERROR, irq_stat_count_show);
>  #endif
>  	return 0;
>  }
>  late_initcall(irq_init_stats);
>  
> +/*
> + * Used for default enabled counters to increment the stats and to enable the
> + * entry for /proc/interrupts output.
> + */
> +void irq_stat_inc_and_enable(enum irq_stat_counts which)
> +{
> +	this_cpu_inc(irq_stat.counts[which]);
> +	set_bit(which, irq_stat_count_show);
> +}
> +
>  #ifdef CONFIG_PROC_FS
>  /*
>   * /proc/interrupts printing for arch specific interrupts
> @@ -159,7 +177,7 @@ int arch_show_interrupts(struct seq_file
>  	const struct irq_stat_info *info = irq_stat_info;
>  
>  	for (unsigned int i = 0; i < ARRAY_SIZE(irq_stat_info); i++, info++) {
> -		if (info->skip_vector)
> +		if (!test_bit(i, irq_stat_count_show))
>  			continue;
>  
>  		seq_printf(p, "%*s:", prec, info->symbol);

Reviewed-by: Radu Rendec <radu@rendec.net>