qmi_wwan: Increase headroom for QMAP SKBs

qmi_wwan: Increase headroom for QMAP SKBs

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@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Link: https://lore.kernel.org/r/20210106122403.1321180-1-kristian.evensen@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>