Re: [PATCH 1/2] softirq: fix integer overflow in function show_stat()

["Leizhen (ThunderTown)" <thunder.leizhen@huaweicloud.com>]

On 2023/7/25 23:26, Matthew Wilcox wrote:
> On Tue, Jul 25, 2023 at 05:09:05PM +0800, Leizhen (ThunderTown) wrote:
>> On 2023/7/25 10:00, Leizhen (ThunderTown) wrote:
>>> On 2023/7/24 21:50, Matthew Wilcox wrote:
>>>> On Mon, Jul 24, 2023 at 09:22:23PM +0800, thunder.leizhen@huaweicloud.com wrote:
>>>>> From: Zhen Lei <thunder.leizhen@huawei.com>
>>>>>
>>>>> The statistics function of softirq is supported by commit aa0ce5bbc2db
>>>>> ("softirq: introduce statistics for softirq") in 2009. At that time,
>>>>> 64-bit processors should not have many cores and would not face
>>>>> significant count overflow problems. Now it's common for a processor to
>>>>> have hundreds of cores. Assume that there are 100 cores and 10
>>>>> TIMER_SOFTIRQ are generated per second, then the 32-bit sum will be
>>>>> overflowed after 50 days.
>>>>
>>>> 50 days is long enough to take a snapshot.  You should always be using
>>>> difference between, not absolute values, and understand that they can
>>>> wrap.  We only tend to change the size of a counter when it can wrap
>>>> sufficiently quickly that we might miss a wrap (eg tens of seconds).
>>
>> Sometimes it can take a long time to view it again. For example, it is
>> possible to run a complete business test for hours or even days, and
>> then calculate the average.
> 
> I've been part of teams which have done such multi-hour tests.  That
> isn't how monitoring was performed.  Instead snapshots were taken every
> minute or even more frequently, because we wanted to know how these
> counters were fluctuating during the test -- were there time periods
> when the number of sortirqs spiked, or was it constant during the test?
> 
>>> Yes, I think patch 2/2 can be dropped. I reduced the number of soft
>>> interrupts generated in one second, and actually 100+ or 1000 is normal.
>>> But I think patch 1/2 is necessary. The sum of the output scattered values
>>> does not match the output sum. To solve this problem, we only need to
>>> adjust the type of a local variable.
>>
>> However, it is important to consider that when the local variable is changed
>> to u64, the output string becomes longer. It is not clear if the user-mode
>> program parses it only by u32.
> 
> There's no need for the numbers to add up.  They won't anyway, because
> summing them is racy , so they'll always be a little off.

Okay, thanks for the reply. I got it. I just summed it up temporarily to
prove that integer overflow is possible, and there's no actual requirement.

> 
> .
> 

-- 
Regards,
  Zhen Lei