netdev - Re: [RFC PATCH net-next 0/6] implement kthread based napi poll

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Message-ID: <20200925120028.65b5cd6d@hermes.lan>
Date:   Fri, 25 Sep 2020 12:00:28 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Wei Wang <weiwan@...gle.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Felix Fietkau <nbd@....name>,
        Björn Töpel <bjorn.topel@...el.com>
Subject: Re: [RFC PATCH net-next 0/6] implement kthread based napi poll

On Fri, 25 Sep 2020 20:23:37 +0200
Eric Dumazet <edumazet@...gle.com> wrote:

> On Fri, Sep 25, 2020 at 8:16 PM Stephen Hemminger
> <stephen@...workplumber.org> wrote:
> >
> > On Fri, 25 Sep 2020 10:15:25 -0700
> > Wei Wang <weiwan@...gle.com> wrote:
> >  
> > > > > In terms of performance, I ran tcp_rr tests with 1000 flows with
> > > > > various request/response sizes, with RFS/RPS disabled, and compared
> > > > > performance between softirq vs kthread. Host has 56 hyper threads and
> > > > > 100Gbps nic.  
> >
> > It would be good to similar tests on othere hardware. Not everyone has
> > server class hardware. There are people running web servers on untuned
> > servers over 10 years old; this may cause a regression there.
> >
> > Not to mention the slower CPU's in embedded systems. How would this
> > impact OpenWrt or Android?  
> 
> Most probably you won't notice a significant difference.
> 
> Switching to a kthread is quite cheap, since you have no MMU games to play with.

That makes sense, and in the past when doing stress tests the napi
work was mostly on the kthread already.

> >
> > Another potential problem is that if you run real time (SCH_FIFO)
> > threads they have higher priority than kthread. So for that use
> > case, moving networking to kthread would break them.  
> 
> Sure, playing with FIFO threads is dangerous.
> 
> Note that our plan is still to have softirqs by default.
> 
> If an admin chose to use kthreads, it is its choice, not ours.
> 
> This is also why I very much prefer the kthread approach to the work
> queue, since the work queue could not be fine tuned.

Agree with you, best to keep this as opt-in.