* Nathan Sullivan | 2015-05-01 10:04:12 [-0500]: >Hello all, > >We are running 3.14.37-rt on a Xilinx Zynq based board, and have noticed some >unfortunate behavior with NAPI polling during heavy incoming traffic. Since, >as I understand it, softirqs are scheduled on the thread that caused them in >rt, the netowrk RX softirq simply runs over and over on one CPU of the system. >The network device never re-enables interupts, basically NAPI polling runs >forever and weight/budget are irrelevant with preempt-rt on. > >Since we set IRQ affinity to CPU 0 for everything, this leads to the system >live-locking and becoming unusable. With full RT preemption off, things are >fine. In addition, 3.2 kernels with RT are fine as well under heavy net load. >Is this behavior due to a design tradeoff, or is it a bug? The rx-napi softirq runs once the threaded handler is done with the handler. Comparing with vanilla there a little difference: vanilla repeats the softirq a number of times and has a time budget. Once it decides that it runs for too long it moves into softirqd. -RT on the other hand repeats the softirq processing as long as there is work to do. Since it is done after the thread handler completes it work it is done at the priority of the threaded handler - so if your -RT task has a higher priority it won't be disturbed by it. If you put your shell above the threaded handler of the network handler (or the handler as SCHED_OTHER) then things should be back to normal. I'm not sure if moving network (after a while of processing) to ksoftirq is a good idea because we lose the priority then. Sebastian -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
