linux-kernel - Re: [PATCH V2 1/1] net: cdc_ncm: Reduce memory use when kernel memory low

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Date:   Mon, 03 Jul 2017 01:51:10 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     jim_baxter@...tor.com
Cc:     linux-usb@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, oliver@...kum.org, bjorn@...k.no,
        David.Laight@...LAB.COM
Subject: Re: [PATCH V2 1/1] net: cdc_ncm: Reduce memory use when kernel
 memory low

From: Jim Baxter <jim_baxter@...tor.com>
Date: Wed, 28 Jun 2017 21:35:29 +0100

> 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.
> Each time this issue occurs we wait for an increasing number of
> reduced size allocations before requesting a full size one to not
> put additional pressure on a low memory system.
> 
> Once the memory is compacted the CDC-NCM data can resume transmitting
> at the normal tx_max rate once again.
> 
> Signed-off-by: Jim Baxter <jim_baxter@...tor.com>

Patch applied, thanks.