Kristian Evensen <kristian.even...@gmail.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.even...@gmail.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 <bj...@mork.no>
