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

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Date:   Tue, 16 May 2017 20:24:10 +0200
From:   Bjørn Mork <bjorn@...k.no>
To:     Jim Baxter <jim_baxter@...tor.com>
Cc:     <linux-usb@...r.kernel.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, Oliver Neukum <oliver@...kum.org>
Subject: Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory low

Jim Baxter <jim_baxter@...tor.com> 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




Bjørn