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

[Felix Fietkau]

On 25.03.23 04:19, Jakub Kicinski wrote:
> On Fri, 24 Mar 2023 18:57:03 +0100 Felix Fietkau wrote:
> > >> 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.
>
> I tried the threaded NAPI on larger systems and helped others try,
> and so far it's not been beneficial :( Even the load balancing
> improvements are not significant enough to use it, and there
> is a large risk of scheduler making the wrong decision.
>
> Hence my questioning - I'm trying to understand what you're doing
> differently.
I didn't actually run any tests on bigger systems myself, so I don't 
know how to tune it for those.

> > >> 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/
>
> Could you provide the full configuration for this test?
> In non-threaded mode the RPS is enabled to spread over remaining
> 3 cores?
In this test I'm using threaded NAPI and backlog_threaded without any 
fixed core assignment.

- Felix