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. > 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@xxxxxxxxxx> 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 <bjorn@xxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
