Re: [RFC 1/2] softirq: Defer net rx/tx processing to ksoftirqd context

[Mike Galbraith]

On Thu, 2018-01-11 at 12:22 -0800, Linus Torvalds wrote:
> On Thu, Jan 11, 2018 at 12:16 PM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > 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.
> 
> Yes.
> 
> So I think one solution might be to have a hybrid system, where we do
> the softirq's synchronously normally (which is what you really want
> for good latency).
> 
> But then fall down on a threaded model - but that fallback case should
> be per-softirq, not global. So if one softirq uses a lot of CPU time,
> that shouldn't affect the latency of other softirqs.
> 
> So maybe we could get rid of the per-cpu ksoftirqd entirely, and
> replace it with with per-cpu and per-softirq workqueues?

How would that be better than what RT used to do, and I still do for my
RT kernels via boot option, namely split ksoftirqd into per-softirq
threads.

	-Mike