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

[David Miller]

From: Bjørn Mork <bjorn@xxxxxxx>
Date: Tue, 16 May 2017 20:24:10 +0200

> 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

When there isn't memory pressure this will hurt performance of
course.

It is a quite common paradigm to back down to 0 order memory requests
when higher order ones fail, so this isn't such a bad change from the
perspective.

However, one negative about it is that when the system is under memory
stress it doesn't help at all to keep attemping high order allocations
when the system hasn't recovered yet.  In fact, this can make it
worse.