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Date:   Fri, 2 Dec 2022 13:39:02 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Etienne Champetier <champetier.etienne@gmail.com>
Cc:     netdev@vger.kernel.org
Subject: Re: Multicast packet reordering
Message-ID: <20221202133902.7888c0ce@kernel.org>
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On Fri, 2 Dec 2022 15:09:13 -0500 Etienne Champetier wrote:
> > Yes, there are per-cpu queues in various places to help scaling,
> > if you don't pin the sender to one CPU and it gets moved you can
> > understandably get reordering w/ UDP (both on lo and veth).  
> 
> Is enabling RPS a workaround that will continue to work in the long term,
> or it just fixes this reordering "by accident" ?

For lo and veth it should continue to work.

> And I guess pinning the sender to one CPU is also important when
> sending via a real NIC, not only moving packets internally ?

Yes, for UDP with real NICs you may want to try to pin a flow 
to a particular NIC queue.. somehow. Not sure how much of a problem
this is in practice.