On Thu, 10 Aug 2017 00:29:19 +0200
Francois Romieu <rom...@fr.zoreil.com> wrote:
> Murali Karicheri <m-kariche...@ti.com> :
> > The internal memory or FIFO can store only up to 3 MTU sized packets. So
> > that has to
> > be processed before PRU gets another packets to send to CPU. So per above,
> > it is not ideal to run NAPI for this scenario, right? Also for NetCP we use
> > about 128 descriptors with MTU size buffers to handle 1Gbps Ethernet link.
> > Based on that roughly we would need at least 10-12 buffers in the FIFO.
> > Currently we have a NAPI implementation in use that gives throughput of
> > 95Mbps for
> > MTU sized packets, but our UDP iperf tests shows less than 1% packet loss
> > for an
> > offered traffic of 95Mbps with MTU sized packets. This is not good for
> > industrial
> > network using HSR/PRP protocol for network redundancy. We need to have zero
> > packet
> > loss for MTU sized packets at 95Mbps throughput. That is the problem
> > description.
> Imvho you should instrument the kernel to figure where the excess latency that
> prevents NAPI processing to take place within 125 us of physical packet
> comes from.
> > As an experiment, I have moved the packet processing to irq handler to see
> > if we
> > can take advantage of CPU cycle to processing the packet instead of NAPI
> > and to check if the firmware encounters buffer overflow. The result is
> > positive
> > with no buffer overflow seen at the firmware and no packet loss in the
> > iperf test.
> > But we want to do more testing as an experiment and ran into a uart console
> > locks
> > up after running traffic for about 2 minutes. So I tried enabling the DEBUG
> > HACK
> > options to get some clue on what is happening and ran into the trace I
> > shared
> > earlier. So what function can I use to allocate SKB from interrupt handler
> > ?
> Is your design also so tight on memory that you can't even refill your own
> software skb pool from some non-irq context then only swap buffers in the
> irq handler ?
The current best practice in network drivers is to receive into
an allocated page, then create skb meta data with build_skb() in the NAPI poll