Re: [patch V3 01/14] x86/irq: Optimize interrupts decimals printing

[David Laight]

On Thu, 26 Mar 2026 22:56:28 +0100
Thomas Gleixner <tglx@xxxxxxxxxx> wrote:

> From: Dmitry Ilvokhin <d@xxxxxxxxxxxx>
> 
> Monitoring tools periodically scan /proc/interrupts to export metrics as a
> timeseries for future analysis and investigation.
> 
> In large fleets, /proc/interrupts is polled (often every few seconds) on
> every machine. The cumulative overhead adds up quickly across thousands
> of nodes, so reducing the cost of generating these stats does have a
> measurable operational impact. With the ongoing trend toward higher core
> counts per machine, this cost becomes even more noticeable over time,
> since interrupt counters are per-CPU. In Meta's fleet, we have observed
> this overhead at scale.
> 
> Although a binary /proc interface would be a better long-term solution
> due to lower formatting (kernel side) and parsing (userspace side)
> overhead, the text interface will remain in use for some time, even if
> better solutions will be available. Optimizing the /proc/interrupts
> printing code is therefore still beneficial.
> 
> Function seq_printf() supports rich format string for decimals printing,
> but it doesn't required for printing /proc/interrupts per CPU counters,
> seq_put_decimal_ull_width() function can be used instead to print per
> CPU counters, because very limited formatting is required for this case.
> Similar optimization idea is already used in show_interrupts().

Did you consider optimising the underlying snprintf() code instead?
I looked at it recently and it looks like it could be optimised somewhat. 

> As a side effect this aligns the x86 decriptions with the generic
> interrupts event descriptions.
> 
> Performance counter stats (truncated) for 'sh -c cat /proc/interrupts
> 
> Before:
> 
>       3.42 msec task-clock        #    0.802 CPUs utilized   ( +-  0.05% )
>          1      context-switches  #  291.991 /sec            ( +-  0.74% )
>          0      cpu-migrations    #    0.000 /sec
>        343      page-faults       #  100.153 K/sec           ( +-  0.01% )
>  8,932,242      instructions      #    1.66  insn per cycle  ( +-  0.34% )
>  5,374,427      cycles            #    1.569 GHz             ( +-  0.04% )
>  1,483,154      branches          #  433.068 M/sec           ( +-  0.22% )
>     28,768      branch-misses     #    1.94% of all branches ( +-  0.31% )
> 
> 0.00427182 +- 0.00000215 seconds time elapsed  ( +-  0.05% )
> 
> After:
> 
>       2.39 msec task-clock        #    0.796 CPUs utilized   ( +-  0.06% )
>          1      context-switches  #  418.541 /sec            ( +-  0.70% )
>          0      cpu-migrations    #    0.000 /sec
>        343      page-faults       #  143.560 K/sec           ( +-  0.01% )
>  7,020,982      instructions      #    1.30  insn per cycle  ( +-  0.52% )
>  5,397,266      cycles            #    2.259 GHz             ( +-  0.06% )
>  1,569,648      branches          #  656.962 M/sec           ( +-  0.08% )
>     25,419      branch-misses     #    1.62% of all branches ( +-  0.72% )
> 
> 0.00299996 +- 0.00000206 seconds time elapsed  ( +-  0.07% )
> 
> Relative speed up in time elapsed is around 29%.

I'm not sure I completely understand that table.
'cycles' seems to have gone up - which ought to make it slower.
But 'GHz' has gone up considerable - which will reduce 'wall time' for
a fixed number of cycles.

Are you sure that you've not just make the cpu clock faster?
It can be hard to lock the clock frequency of Intel cpu in order to
get meaningful benchmark times.

It also only really makes sense to measure how long the read() itself takes.
Everything else is unwanted noise as far as this change is concerned.

	David

> 
> [ tglx: Fixed it up so it applies to current mainline ]
> 
> Signed-off-by: Dmitry Ilvokhin <d@xxxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Reviewed-by: Radu Rendec <radu@xxxxxxxxxx>
> Link: https://patch.msgid.link/aQj5mGZ6_BBlAm3B@xxxxxxxxxxxxxxxxxx
> 
> ---
> Changes v2:
> - Expanded commit message: add more rationale for the proposed change.
> - Renamed helper put_spaced_decimal() -> put_decimal() primarely to make
>   checkpatch.pl --strict pass.
> 
>  arch/x86/kernel/irq.c |  112 ++++++++++++++++++++++++++------------------------
>  1 file changed, 59 insertions(+), 53 deletions(-)
> --- a/arch/x86/kernel/irq.c
> +++ b/arch/x86/kernel/irq.c
> @@ -62,6 +62,18 @@ void ack_bad_irq(unsigned int irq)
>  	apic_eoi();
>  }
>  
> +/*
> + * A helper routine for putting space and decimal number without overhead
> + * from rich format of printf().
> + */
> +static void put_decimal(struct seq_file *p, unsigned long long num)
> +{
> +	const char *delimiter = " ";
> +	unsigned int width = 10;
> +
> +	seq_put_decimal_ull_width(p, delimiter, num, width);
> +}
> +
>  #define irq_stats(x)		(&per_cpu(irq_stat, x))
>  /*
>   * /proc/interrupts printing for arch specific interrupts
> @@ -70,103 +82,101 @@ int arch_show_interrupts(struct seq_file
>  {
>  	int j;
>  
> -	seq_printf(p, "%*s: ", prec, "NMI");
> +	seq_printf(p, "%*s:", prec, "NMI");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->__nmi_count);
> +		put_decimal(p, irq_stats(j)->__nmi_count);
>  	seq_puts(p, "  Non-maskable interrupts\n");
>  #ifdef CONFIG_X86_LOCAL_APIC
> -	seq_printf(p, "%*s: ", prec, "LOC");
> +	seq_printf(p, "%*s:", prec, "LOC");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->apic_timer_irqs);
> +		put_decimal(p, irq_stats(j)->apic_timer_irqs);
>  	seq_puts(p, "  Local timer interrupts\n");
>  
> -	seq_printf(p, "%*s: ", prec, "SPU");
> +	seq_printf(p, "%*s:", prec, "SPU");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->irq_spurious_count);
> +		put_decimal(p, irq_stats(j)->irq_spurious_count);
>  	seq_puts(p, "  Spurious interrupts\n");
> -	seq_printf(p, "%*s: ", prec, "PMI");
> +	seq_printf(p, "%*s:", prec, "PMI");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->apic_perf_irqs);
> +		put_decimal(p, irq_stats(j)->apic_perf_irqs);
>  	seq_puts(p, "  Performance monitoring interrupts\n");
> -	seq_printf(p, "%*s: ", prec, "IWI");
> +	seq_printf(p, "%*s:", prec, "IWI");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->apic_irq_work_irqs);
> +		put_decimal(p, irq_stats(j)->apic_irq_work_irqs);
>  	seq_puts(p, "  IRQ work interrupts\n");
> -	seq_printf(p, "%*s: ", prec, "RTR");
> +	seq_printf(p, "%*s:", prec, "RTR");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->icr_read_retry_count);
> +		put_decimal(p, irq_stats(j)->icr_read_retry_count);
>  	seq_puts(p, "  APIC ICR read retries\n");
>  	if (x86_platform_ipi_callback) {
> -		seq_printf(p, "%*s: ", prec, "PLT");
> +		seq_printf(p, "%*s:", prec, "PLT");
>  		for_each_online_cpu(j)
> -			seq_printf(p, "%10u ", irq_stats(j)->x86_platform_ipis);
> +			put_decimal(p, irq_stats(j)->x86_platform_ipis);
>  		seq_puts(p, "  Platform interrupts\n");
>  	}
>  #endif
>  #ifdef CONFIG_SMP
> -	seq_printf(p, "%*s: ", prec, "RES");
> +	seq_printf(p, "%*s:", prec, "RES");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->irq_resched_count);
> +		put_decimal(p, irq_stats(j)->irq_resched_count);
>  	seq_puts(p, "  Rescheduling interrupts\n");
> -	seq_printf(p, "%*s: ", prec, "CAL");
> +	seq_printf(p, "%*s:", prec, "CAL");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->irq_call_count);
> +		put_decimal(p, irq_stats(j)->irq_call_count);
>  	seq_puts(p, "  Function call interrupts\n");
> -	seq_printf(p, "%*s: ", prec, "TLB");
> +	seq_printf(p, "%*s:", prec, "TLB");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->irq_tlb_count);
> +		put_decimal(p, irq_stats(j)->irq_tlb_count);
>  	seq_puts(p, "  TLB shootdowns\n");
>  #endif
>  #ifdef CONFIG_X86_THERMAL_VECTOR
> -	seq_printf(p, "%*s: ", prec, "TRM");
> +	seq_printf(p, "%*s:", prec, "TRM");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->irq_thermal_count);
> +		put_decimal(p, irq_stats(j)->irq_thermal_count);
>  	seq_puts(p, "  Thermal event interrupts\n");
>  #endif
>  #ifdef CONFIG_X86_MCE_THRESHOLD
> -	seq_printf(p, "%*s: ", prec, "THR");
> +	seq_printf(p, "%*s:", prec, "THR");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->irq_threshold_count);
> +		put_decimal(p, irq_stats(j)->irq_threshold_count);
>  	seq_puts(p, "  Threshold APIC interrupts\n");
>  #endif
>  #ifdef CONFIG_X86_MCE_AMD
> -	seq_printf(p, "%*s: ", prec, "DFR");
> +	seq_printf(p, "%*s:", prec, "DFR");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->irq_deferred_error_count);
> +		put_decimal(p, irq_stats(j)->irq_deferred_error_count);
>  	seq_puts(p, "  Deferred Error APIC interrupts\n");
>  #endif
>  #ifdef CONFIG_X86_MCE
> -	seq_printf(p, "%*s: ", prec, "MCE");
> +	seq_printf(p, "%*s:", prec, "MCE");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", per_cpu(mce_exception_count, j));
> +		put_decimal(p, per_cpu(mce_exception_count, j));
>  	seq_puts(p, "  Machine check exceptions\n");
> -	seq_printf(p, "%*s: ", prec, "MCP");
> +	seq_printf(p, "%*s:", prec, "MCP");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", per_cpu(mce_poll_count, j));
> +		put_decimal(p, per_cpu(mce_poll_count, j));
>  	seq_puts(p, "  Machine check polls\n");
>  #endif
>  #ifdef CONFIG_X86_HV_CALLBACK_VECTOR
>  	if (test_bit(HYPERVISOR_CALLBACK_VECTOR, system_vectors)) {
> -		seq_printf(p, "%*s: ", prec, "HYP");
> +		seq_printf(p, "%*s:", prec, "HYP");
>  		for_each_online_cpu(j)
> -			seq_printf(p, "%10u ",
> -				   irq_stats(j)->irq_hv_callback_count);
> +			put_decimal(p, irq_stats(j)->irq_hv_callback_count);
>  		seq_puts(p, "  Hypervisor callback interrupts\n");
>  	}
>  #endif
>  #if IS_ENABLED(CONFIG_HYPERV)
>  	if (test_bit(HYPERV_REENLIGHTENMENT_VECTOR, system_vectors)) {
> -		seq_printf(p, "%*s: ", prec, "HRE");
> +		seq_printf(p, "%*s:", prec, "HRE");
>  		for_each_online_cpu(j)
> -			seq_printf(p, "%10u ",
> -				   irq_stats(j)->irq_hv_reenlightenment_count);
> +			put_decimal(p,
> +				    irq_stats(j)->irq_hv_reenlightenment_count);
>  		seq_puts(p, "  Hyper-V reenlightenment interrupts\n");
>  	}
>  	if (test_bit(HYPERV_STIMER0_VECTOR, system_vectors)) {
> -		seq_printf(p, "%*s: ", prec, "HVS");
> +		seq_printf(p, "%*s:", prec, "HVS");
>  		for_each_online_cpu(j)
> -			seq_printf(p, "%10u ",
> -				   irq_stats(j)->hyperv_stimer0_count);
> +			put_decimal(p, irq_stats(j)->hyperv_stimer0_count);
>  		seq_puts(p, "  Hyper-V stimer0 interrupts\n");
>  	}
>  #endif
> @@ -175,35 +185,31 @@ int arch_show_interrupts(struct seq_file
>  	seq_printf(p, "%*s: %10u\n", prec, "MIS", atomic_read(&irq_mis_count));
>  #endif
>  #if IS_ENABLED(CONFIG_KVM)
> -	seq_printf(p, "%*s: ", prec, "PIN");
> +	seq_printf(p, "%*s:", prec, "PIN");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ", irq_stats(j)->kvm_posted_intr_ipis);
> +		put_decimal(p, irq_stats(j)->kvm_posted_intr_ipis);
>  	seq_puts(p, "  Posted-interrupt notification event\n");
>  
> -	seq_printf(p, "%*s: ", prec, "NPI");
> +	seq_printf(p, "%*s:", prec, "NPI");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ",
> -			   irq_stats(j)->kvm_posted_intr_nested_ipis);
> +		put_decimal(p, irq_stats(j)->kvm_posted_intr_nested_ipis);
>  	seq_puts(p, "  Nested posted-interrupt event\n");
>  
> -	seq_printf(p, "%*s: ", prec, "PIW");
> +	seq_printf(p, "%*s:", prec, "PIW");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ",
> -			   irq_stats(j)->kvm_posted_intr_wakeup_ipis);
> +		put_decimal(p, irq_stats(j)->kvm_posted_intr_wakeup_ipis);
>  	seq_puts(p, "  Posted-interrupt wakeup event\n");
>  #endif
>  #ifdef CONFIG_GUEST_PERF_EVENTS
> -	seq_printf(p, "%*s: ", prec, "VPMI");
> +	seq_printf(p, "%*s:", prec, "VPMI");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ",
> -			   irq_stats(j)->perf_guest_mediated_pmis);
> +		put_decimal(p, irq_stats(j)->perf_guest_mediated_pmis);
>  	seq_puts(p, " Perf Guest Mediated PMI\n");
>  #endif
>  #ifdef CONFIG_X86_POSTED_MSI
> -	seq_printf(p, "%*s: ", prec, "PMN");
> +	seq_printf(p, "%*s:", prec, "PMN");
>  	for_each_online_cpu(j)
> -		seq_printf(p, "%10u ",
> -			   irq_stats(j)->posted_msi_notification_count);
> +		put_decimal(p, irq_stats(j)->posted_msi_notification_count);
>  	seq_puts(p, "  Posted MSI notification event\n");
>  #endif
>  	return 0;
> 
>