netdev - Re: [PATCH net-next 0/2] net: Use SMP threads for backlog NAPI (or optional).

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Date: Tue, 31 Oct 2023 08:36:14 -0300
From: Wander Lairson Costa <wander@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Jakub Kicinski <kuba@...nel.org>, juri.lelli@...hat.com, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Come On Now <hawk@...nel.org>
Subject: Re: [PATCH net-next 0/2] net: Use SMP threads for backlog NAPI (or optional).

On Tue, Oct 31, 2023 at 7:14 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2023-10-16 16:53:39 [+0200], To Jakub Kicinski wrote:
> > On 2023-10-16 07:17:56 [-0700], Jakub Kicinski wrote:
> > > On Mon, 16 Oct 2023 11:53:21 +0200 Sebastian Andrzej Siewior wrote:
> > > > > Do we have reason to believe nobody uses RPS?
> > > >
> > > > Not sure what you relate to. I would assume that RPS is used in general
> > > > on actual devices and not on loopback where backlog is used. But it is
> > > > just an assumption.
> > > > The performance drop, which I observed with RPS and stress-ng --udp, is
> > > > within the same range with threads and IPIs (based on memory). I can
> > > > re-run the test and provide actual numbers if you want.
> > >
> > > I was asking about RPS because with your current series RPS processing
> > > is forced into threads. IDK how well you can simulate the kind of
> > > workload which requires RPS. I've seen it used mostly on proxyies
> > > and gateways. For proxies Meta's experiments with threaded NAPI show
> > > regressions across the board. So "force-threading" RPS will most likely
> > > also cause regressions.
> >
> > Understood.
> >
> > Wandere/ Juri: Do you have any benchmark/ workload where you would see
> > whether RPS with IPI (now) vs RPS (this patch) shows any regression?
>
> So I poked offlist other RH people and I've been told that they hardly
> ever test RPS since the NICs these days have RSS in hardware.

Sorry, Juri is in PTO and I am just back from sick leave and still
catching up. I've been contacting some QE people, but so far it is
like you said, no stress test for RPS. If I have some news, I let you
know.


>
> Sebastian
>