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

[Matthew Wilcox]

On Mon, Jul 24, 2023 at 09:22:23PM +0800, thunder.leizhen@xxxxxxxxxxxxxxx wrote:
> From: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> 
> 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).