netdev - Re: Modification to skb->queue_mapping affecting performance

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Date:   Tue, 13 Sep 2016 16:07:01 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Michael Ma <make0818@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Modification to skb->queue_mapping affecting performance

On Tue, 2016-09-13 at 15:59 -0700, Michael Ma wrote:
> Hi -
> 
> We currently use mqprio on ifb to work around the qdisc root lock
> contention on the receiver side. The problem that we found was that
> queue_mapping is already set when redirecting from ingress qdisc to
> ifb (based on RX selection, I guess?) so the TX queue selection is not
> based on priority.
> 
> Then we implemented a filter which can set skb->queue_mapping to 0 so
> that TX queue selection can be done as expected and flows with
> different priorities will go through different TX queues. However with
> the queue_mapping recomputed, we found the achievable bandwidth with
> small packets (512 bytes) dropped significantly if they're targeting
> different queues. From perf profile I don't see any bottleneck from
> CPU perspective.
> 
> Any thoughts on why modifying queue_mapping will have this kind of
> effect? Also is there any better way of achieving receiver side
> throttling using HTB while avoiding the qdisc root lock on ifb?

But, how many queues do you have on your NIC, and have you setup ifb to
have a same number of queues ?

There is no qdisc lock contention anymore AFAIK, since each cpu will use
a dedicate IFB queue and tasklet.