Re: [PATCHv10 3/4] genirq: Avoid summation loops for /proc/interrupts

[Bitao Hu <yaoma@linux.alibaba.com>]

Hi,

On 2024/2/27 17:26, Thomas Gleixner wrote:
> On Mon, Feb 26 2024 at 10:09, Bitao Hu wrote:
>> We could use the irq_desc::tot_count member to avoid the summation
>> loop for interrupts which are not marked as 'PER_CPU' interrupts in
>> 'show_interrupts'. This could reduce the time overhead of reading
>> /proc/interrupts.
> 
> "Could" is not really a technical term. Either we do or we do not. Also
> please provide context for your change and avoid the 'We'.
OK.
> 
>> --- a/include/linux/irqdesc.h
>> +++ b/include/linux/irqdesc.h
>> @@ -121,6 +121,8 @@ static inline void irq_unlock_sparse(void) { }
>>   extern struct irq_desc irq_desc[NR_IRQS];
>>   #endif
>>
>> +extern bool irq_is_nmi(struct irq_desc *desc);
>> +
> 
> If at all this wants to be in kernel/irq/internal.h. There is zero
> reason to expose this globally.
> 
>> -static bool irq_is_nmi(struct irq_desc *desc)
>> +bool irq_is_nmi(struct irq_desc *desc)
>>   {
>>   	return desc->istate & IRQS_NMI;
>>   }
> 
> If at all this really wants to be a static inline in internals.h, but
> instead of blindly copying code this can be done smarter:
> 
> unsigned int kstat_irq_desc(struct irq_desc *desc)
> {
> 	unsigned int sum = 0;
> 	int cpu;
> 
> 	if (!irq_settings_is_per_cpu_devid(desc) &&
> 	    !irq_settings_is_per_cpu(desc) &&
> 	    !irq_is_nmi(desc))
> 		return data_race(desc->tot_count);
> 
> 	for_each_possible_cpu(cpu)
> 		sum += data_race(*per_cpu_ptr(desc->kstat_irqs, cpu));
> 	return sum;
> }
> 
> and then let kstat_irqs() and show_interrupts() use it. See?

I have a concern. kstat_irqs() uses for_each_possible_cpu() for
summation. However, show_interrupts() uses for_each_online_cpu(),
which means it only outputs interrupt statistics for online cpus.
If we use for_each_possible_cpu() in show_interrupts() to calculate
'any_count', there could be a problem with the following scenario:
If an interrupt has a count of zero on online cpus but a non-zero
count on possible cpus, then 'any_count' would not be zero, and the
statistics for that interrupt would be output, which is not the
desired behavior for show_interrupts(). Therefore, I think it's not
good to have kstat_irqs() and show_interrupts() both use the same
logic. What do you think?

> 
> With that a proper changelog would be:
> 
>     show_interrupts() unconditionally accumulates the per CPU interrupt
>     statistics to determine whether an interrupt was ever raised.
> 
>     This can be avoided for all interrupts which are not strictly per CPU
>     and not of type NMI because those interrupts provide already an
>     accumulated counter. The required logic is already implemented in
>     kstat_irqs().
> 
>     Split the inner access logic out of kstat_irqs() and use it for
>     kstat_irqs() and show_interrupts() to avoid the accumulation loop
>     when possible.
> 

Best Regards,
	Bitao Hu