> When CPU2 is off-lined, the statistics for CPU2 do not > appear (expected). However when you look at the before > picture (all CPUs present) and after picture (all cpus > present after CPU2 re-added), you see that the original > data was returned and has incremented: <snip> > Results are similar for other interrupt levels. > > This does not seem like the correct approach. Shouldn't an > added CPU be considered new and stats begin again from zero? > > Can you please give us guidance on this issue? Is it > a bug or expected behavior? > There are several legitimate options here, including the current behavior. I'm not clear other than kernel developers doing debugging who consumes this data. If it is just kernel developers I contend we are capable of understanding whatever data is presented. Current Behavior: Stats carry over. The advantage of this is that the stats history is not lost. Consider the scenario where a cpu is moved regularly between two partitions in response to workload. It would probably be best in that situation to keep the full history of that cpu during all the times it has been in the partition. This view would be consistent with the cpu always being there, just sometimes being off. I haven't fully thought through the impact of a different physical cpu being added in the same logical slot, or how virtualized cpus fits into this paradigm. But I suspect they shouldn't change the concept. Reset every time: The advantage of this is the cpus start with a clean slate. There is logic to avoiding the situation where some hotplug added cpus have state already and some hotplug added cpus start with nothing. Consistency is not a bad thing. On the other hand this is a bit of selective amnesia, we would be forgetting all past statistics only for cpus that had taken some time off. It's like taking a vacation from work and having your old email thrown away while you are gone. Reset all every time: It could well be argued that the old cpu statistics from every active cpu serve only to confuse things because the cpus were in a different state. If you didn't reset all cpus statistics, then after adding a new cpu you couldn't say for instance that cpu 5 is taking 45% of the interrupts. The statistics are meaningless without their context, and by adding a cpu we have destroyed their context. Therefore we should reset all statistics so they all have the same frame of reference. The downside of course is that we lose all history on all cpus. Personally, I'm fine with the current behavior. It makes the most sense to me and is the least complex to implement (already done). We maybe should document it (send a patch in reply to the one Ashok Raj just posted to lkml). Unless somebody has a real user of cpu statistics that this behavior breaks.
