net: mvneta: fix Tx interrupt delay
The mvneta driver sets the amount of Tx coalesce packets to 16 by default. Normally that does not cause any trouble since the driver uses a much larger Tx ring size (532 packets). But some sockets might run with very small buffers, much smaller than the equivalent of 16 packets. This is what ping is doing for example, by setting SNDBUF to 324 bytes rounded up to 2kB by the kernel. The problem is that there is no documented method to force a specific packet to emit an interrupt (eg: the last of the ring) nor is it possible to make the NIC emit an interrupt after a given delay. In this case, it causes trouble, because when ping sends packets over its raw socket, the few first packets leave the system, and the first 15 packets will be emitted without an IRQ being generated, so without the skbs being freed. And since the socket's buffer is small, there's no way to reach that amount of packets, and the ping ends up with "send: no buffer available" after sending 6 packets. Running with 3 instances of ping in parallel is enough to hide the problem, because with 6 packets per instance, that's 18 packets total, which is enough to grant a Tx interrupt before all are sent. The original driver in the LSP kernel worked around this design flaw by using a software timer to clean up the Tx descriptors. This timer was slow and caused terrible network performance on some Tx-bound workloads (such as routing) but was enough to make tools like ping work correctly. Instead here, we simply set the packet counts before interrupt to 1. This ensures that each packet sent will produce an interrupt. NAPI takes care of coalescing interrupts since the interrupt is disabled once generated. No measurable performance impact nor CPU usage were observed on small nor large packets, including when saturating the link on Tx, and this fixes tools like ping which rely on too small a send buffer. If one wants to increase this value for certain workloads where it is safe to do so, "ethtool -C $dev tx-frames" will override this default setting. This fix needs to be applied to stable kernels starting with 3.10. Tested-By:Maggie Mae Roxas <maggie.mae.roxas@gmail.com> Signed-off-by:
Willy Tarreau <w@1wt.eu> Signed-off-by:
David S. Miller <davem@davemloft.net>
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