Re: [patch V2 1/2] genriq: Avoid summation loops for /proc/stat

Waiman Long
2019-02-08T22:46:44Z

On 02/08/2019 05:32 PM, Andrew Morton wrote:
> On Fri, 08 Feb 2019 14:48:03 +0100 Thomas Gleixner <tglx@linutronix.de> wrote:
>
>> Waiman reported that on large systems with a large amount of interrupts the
>> readout of /proc/stat takes a long time to sum up the interrupt
>> statistics. In principle this is not a problem. but for unknown reasons
>> some enterprise quality software reads /proc/stat with a high frequency.
>>
>> The reason for this is that interrupt statistics are accounted per cpu. So
>> the /proc/stat logic has to sum up the interrupt stats for each interrupt.
>>
>> This can be largely avoided for interrupts which are not marked as
>> 'PER_CPU' interrupts by simply adding a per interrupt summation counter
>> which is incremented along with the per interrupt per cpu counter.
>>
>> The PER_CPU interrupts need to avoid that and use only per cpu accounting
>> because they share the interrupt number and the interrupt descriptor and
>> concurrent updates would conflict or require unwanted synchronization.
>>
>> ...
>>
>> --- a/include/linux/irqdesc.h
>> +++ b/include/linux/irqdesc.h
>> @@ -65,6 +65,7 @@ struct irq_desc {
>> unsigned int core_internal_state__do_not_mess_with_it;
>> unsigned int depth; /* nested irq disables */
>> unsigned int wake_depth; /* nested wake enables */
>> + unsigned int tot_count;
> Confused. Isn't this going to quickly overflow?
>
>
All the current irq count computations for each individual irqs are
using unsigned int type. Only the sum of all the irqs is u64. Yes, it is
possible for an individual irq count to exceed 32 bits given sufficient
uptime.  My PC has an uptime of 36 days and the highest irq count value
is 79,227,699. Given the current rate, the overflow will happen after
about 5 years. A larger server system may have an overflow in much
shorter period. So maybe we should consider changing all the irq counts
to unsigned long then.

Cheers,
Longman