netdev - Re: [PATCH net-next] qmi_wwan: Increase headroom for QMAP SKBs

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Message-ID: <87ft3e3zo1.fsf@miraculix.mork.no>
Date:   Wed, 06 Jan 2021 15:31:10 +0100
From:   Bjørn Mork <bjorn@...k.no>
To:     Kristian Evensen <kristian.evensen@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next] qmi_wwan: Increase headroom for QMAP SKBs

Kristian Evensen <kristian.evensen@...il.com> writes:

> When measuring the throughput (iperf3 + TCP) while routing on a
> not-so-powerful device (Mediatek MT7621, 880MHz CPU), I noticed that I
> achieved significantly lower speeds with QMI-based modems than for
> example a USB LAN dongle. The CPU was saturated in all of my tests.
>
> With the dongle I got ~300 Mbit/s, while I only measured ~200 Mbit/s
> with the modems. All offloads, etc.  were switched off for the dongle,
> and I configured the modems to use QMAP (16k aggregation). The tests
> with the dongle were performed in my local (gigabit) network, while the
> LTE network the modems were connected to delivers 700-800 Mbit/s.
>
> Profiling the kernel revealed the cause of the performance difference.
> In qmimux_rx_fixup(), an SKB is allocated for each packet contained in
> the URB. This SKB has too little headroom, causing the check in
> skb_cow() (called from ip_forward()) to fail. pskb_expand_head() is then
> called and the SKB is reallocated. In the output from perf, I see that a
> significant amount of time is spent in pskb_expand_head() + support
> functions.
>
> In order to ensure that the SKB has enough headroom, this commit
> increases the amount of memory allocated in qmimux_rx_fixup() by
> LL_MAX_HEADER. The reason for using LL_MAX_HEADER and not a more
> accurate value, is that we do not know the type of the outgoing network
> interface. After making this change, I achieve the same throughput with
> the modems as with the dongle.
>
> Signed-off-by: Kristian Evensen <kristian.evensen@...il.com>

Nice work!

Just wondering: Will the same problem affect the usbnet allocated skbs
as well in case of raw-ip? They will obviously be large enough, but the
reserved headroom probably isn't when we put an IP packet there without
any L2 header?

In any case:

Acked-by: Bjørn Mork <bjorn@...k.no>