Re: [PATCH net-next] net/core: add optional threading for backlog processing

[Paolo Abeni]

On Fri, 2023-03-24 at 18:57 +0100, Felix Fietkau wrote:
> On 24.03.23 18:47, Jakub Kicinski wrote:
> > On Fri, 24 Mar 2023 18:35:00 +0100 Felix Fietkau wrote:
> > > I'm primarily testing this on routers with 2 or 4 CPUs and limited 
> > > processing power, handling routing/NAT. RPS is typically needed to 
> > > properly distribute the load across all available CPUs. When there is 
> > > only a small number of flows that are pushing a lot of traffic, a static 
> > > RPS assignment often leaves some CPUs idle, whereas others become a 
> > > bottleneck by being fully loaded. Threaded NAPI reduces this a bit, but 
> > > CPUs can become bottlenecked and fully loaded by a NAPI thread alone.
> > 
> > The NAPI thread becomes a bottleneck with RPS enabled?
> 
> The devices that I work with often only have a single rx queue. That can
> easily become a bottleneck.
> 
> > > Making backlog processing threaded helps split up the processing work 
> > > even more and distribute it onto remaining idle CPUs.
> > 
> > You'd want to have both threaded NAPI and threaded backlog enabled?
> 
> Yes
> 
> > > It can basically be used to make RPS a bit more dynamic and 
> > > configurable, because you can assign multiple backlog threads to a set 
> > > of CPUs and selectively steer packets from specific devices / rx queues 
> > 
> > Can you give an example?
> > 
> > With the 4 CPU example, in case 2 queues are very busy - you're trying
> > to make sure that the RPS does not end up landing on the same CPU as
> > the other busy queue?
> 
> In this part I'm thinking about bigger systems where you want to have a
> group of CPUs dedicated to dealing with network traffic without
> assigning a fixed function (e.g. NAPI processing or RPS target) to each
> one, allowing for more dynamic processing.
> 
> > > to them and allow the scheduler to take care of the rest.
> > 
> > You trust the scheduler much more than I do, I think :)
> 
> In my tests it brings down latency (both avg and p99) considerably in
> some cases. I posted some numbers here:
> https://lore.kernel.org/netdev/e317d5bc-cc26-8b1b-ca4b-66b5328683c4@xxxxxxxx/

It's still not 110% clear to me why/how this additional thread could
reduce latency. What/which threads are competing for the busy CPU[s]? I
suspect it could be easier/cleaner move away the others (non RPS)
threads.

Cheers,

Paolo