On Thu, Jan 11, 2018 at 12:03 PM, Linus Torvalds <torva...@linux-foundation.org> wrote: > On Thu, Jan 11, 2018 at 11:48 AM, Eric Dumazet <eduma...@google.com> wrote: >> That was the purpose on the last patch : As soon as ksoftirqd is scheduled >> (by some kind of jitter in the 99,000 pps workload, or antagonist wakeup), >> we then switch to a mode where process scheduler can make decisions >> based on threads prios and cpu usage. > > Yeah, but that really screws up everybody else. > > It really is a soft *interrupt*. That was what it was designed for. > The thread handling is not primary, it's literally a fallback to avoid > complete starvation. > > The fact that networking has now - for several years - tried to make > it some kind of thread and get fairness with user threads is all > entirely antithetical to what softirq was designed for. > >> Then, as soon as the load was able to finish in its quantum the >> pending irqs, we re-enter the mode >> where softirq are immediately serviced. > > Except that's not at all how the code works. > > As I pointed out, the softirq thread can be scheduled away, but the > "softiq_running()" wil stilll return true - and the networking code > has now screwed up all the *other* softirqs too! > > I really suspect that what networking wants is more like the > workqueues. Or at least more isolation between different softirq > users, but that's fairly fundamentally hard, given how softirqs are > designed. >
Note that when I implemented TCP Small queues, I did experiments between using a work queue or a tasklet, and workqueues added unacceptable P99 latencies, when many user threads are competing with kernel threads. I suspect that firing a worqueue for networking RX will likely have the same effect :/ Note that the current __do_softirq() implementation suffers from the following : Say we receive NET_RX softirq -> While processing the packet, we wakeup on thread (thus need_resched() becomes true), but also raise a tasklet (because a particular driver needs some extra processing in tasklet context instead of NET_RX ???) -> Then we exit __do_softirq() _and_ schedule ksoftirqd (because tasklet needs to be serviced)
