Hi there, I'd like to understand more in detail how ksoftirqd works in combination with Linux kernel networking.
I'm doing IPSec processing on my Linux machine (kernel 4.4). On one interface I receive encrypted packets which are decrypted and then routed out plaintext (TCP) via another interface. Now I can see that ksoftirqd runs at 100% CPU. Furthermore, I see that initially it runs on core 0, then after a few seconds, it switches over to core 1 and runs at 100%, then switches over to core 2, and so on. I have two questions: 1) I assume it switches cores because the kernel thread is scheduled on different cores, depending on where the softirqs happen. Why does the load never scale across cores? As far as I understand softirqs can run concurrently on different cores. So why does it not scale? 2) Is it normal that ksoftirqd runs at such a high CPU load when doing IPSec, if the core is a small core with small frequency (Intel Atoms, for example)? And if so, is there any way to optimize this for IPSec? 3) When I pin RX queue interrupts of the NIC to certain cores, I can see that interrupts are really only processed on that core. However, it does not change anything about the ksoftirqd behavior. Should it not match, how I route interrupts? Many thanks, Anna
