netdev - Re: [PATCH net-next v2] net/core: add optional threading for backlog processing

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Date:   Thu, 30 Mar 2023 13:01:31 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>, Felix Fietkau <nbd@....name>
Cc:     netdev@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2] net/core: add optional threading for
 backlog processing

On Tue, 2023-03-28 at 16:16 -0700, Jakub Kicinski wrote:
> On Tue, 28 Mar 2023 21:59:25 +0200 Felix Fietkau wrote:
> > When dealing with few flows or an imbalance on CPU utilization, static RPS
> > CPU assignment can be too inflexible. Add support for enabling threaded NAPI
> > for backlog processing in order to allow the scheduler to better balance
> > processing. This helps better spread the load across idle CPUs.
> 
> Can you share some numbers vs a system where RPS only spreads to 
> the cores which are not running NAPI?
> 
> IMHO you're putting a lot of faith in the scheduler and you need 
> to show that it actually does what you say it will do.

I have the same feeling. From your description I think some gain is
possible if there are no other processes running except
ksoftirq/rps/threaded napi. 

I guess that the above is expect average state for a small s/w router,
but if/when routing daemon/igmp proxy/local web server kicks-in you
should notice a measurable higher latency (compared to plain RPS in the
same scenario)???

Cheers,

Paolo