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Date:   Tue, 28 Mar 2023 11:45:32 +0200
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To:     Paolo Abeni <pabeni@redhat.com>, Jakub Kicinski <kuba@kernel.org>
Cc:     netdev@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
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From:   Felix Fietkau <nbd@nbd.name>
Subject: Re: [PATCH net-next] net/core: add optional threading for backlog
 processing
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On 28.03.23 11:29, Paolo Abeni wrote:
> 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@nbd.name/
> 
> 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.
In the tests that I'm doing, network processing load from routing/NAT is 
enough to occupy all available CPUs.
If I dedicate the NAPI thread to one core and use RPS to steer packet 
processing to the other cores, the core taking care of NAPI has some 
idle cycles that go to waste, while the other cores are busy.
If I include the core in the RPS mask, it can take too much away from 
the NAPI thread.
Also, with a smaller number of flows and depending on the hash 
distribution of those flows, there can be an imbalance between how much 
processing each RPS instance gets, making this even more difficult to 
get right.
With the extra threading it works out better in my tests, because the 
scheduler can deal with these kinds of imbalances.

- Felix