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SubjectRe: [PATCH V2 1/1] net: cdc_ncm: Reduce memory use when kernel memory low

> Jim Baxter <> 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.
>> 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 <>
> This looks good to me.
> I would still be happier if we didn't need something like this, but I
> understand that we do. And this patch looks as clean as it can get. I
> haven't tested the patch under any sort of memory pressure, but I did a
> basic runtime test on a single MBIM device. As expected, I did not
> notice any changes with this patch applied.
> But regarding noticable effects: The patch adds no printks, counters or
> sysfs attributes which could tell the user that the initial buffer
> allocation has failed. Maybe add some sort of debug helper(s) in a
> followup patch? How did you verify the patch operation while testing it?
> Anyway, that's no show stopper of course. So FWIW:
> Reviewed-by: Bjørn Mork <>

Hello Bjørn,

I tested this with printk's to show when the low memory code was triggered
and the value of ctx->tx_low_mem_val and ctx->tx_low_mem_max_cnt.
I created a workqueue that slowly used up the atomic memory until the
code is triggered.

I could add debug prints, though I have noticed that cdc_ncm_fill_tx_frame()
does not currently have any debug prints do you think this is because it can be
called in an atomic context and I think debug messages if enabled could cause
too great a delay?


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