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@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). >> >> 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