Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory low

From: BjÃrn Mork
Date: Tue May 16 2017 - 14:24:23 EST

Jim Baxter <jim_baxter@xxxxxxxxxx> writes:

> The CDC-NCM driver can require large amounts of memory to create
> skb's and this can be a problem when the memory becomes fragmented.
> This especially affects embedded systems that have constrained
> resources but wish to maximise the throughput of CDC-NCM with 16KiB
> NTB's.
> The issue is after running for a while the kernel memory can become
> fragmented and it needs compacting.
> If the NTB allocation is needed before the memory has been compacted
> the atomic allocation can fail which can cause increased latency,
> large re-transmissions or disconnections depending upon the data
> being transmitted at the time.
> This situation occurs for less than a second until the kernel has
> compacted the memory but the failed devices can take a lot longer to
> recover from the failed TX packets.
> To ease this temporary situation I modified the CDC-NCM TX path to
> temporarily switch into a reduced memory mode which allocates an NTB
> that will fit into a USB_CDC_NCM_NTB_MIN_OUT_SIZE (default 2048 Bytes)
> sized memory block and only transmit NTB's with a single network frame
> until the memory situation is resolved.
> Once the memory is compacted the CDC-NCM data can resume transmitting
> at the normal tx_max rate once again.

I must say that I don't like the additional complexity added here. If
there are memory issues and you can reduce the buffer size to
USB_CDC_NCM_NTB_MIN_OUT_SIZE, then why don't you just set a lower tx_max
buffer size in the first place?

echo 2048 > /sys/class/net/wwan0/cdc_ncm/tx_max