On 14 Aug 2019, at 0:27, Magnus Karlsson wrote: > This commit adds support for a new flag called need_wakeup in the > AF_XDP Tx and fill rings. When this flag is set, it means that the > application has to explicitly wake up the kernel Rx (for the bit in > the fill ring) or kernel Tx (for bit in the Tx ring) processing by > issuing a syscall. Poll() can wake up both depending on the flags > submitted and sendto() will wake up tx processing only. > > The main reason for introducing this new flag is to be able to > efficiently support the case when application and driver is executing > on the same core. Previously, the driver was just busy-spinning on the > fill ring if it ran out of buffers in the HW and there were none on > the fill ring. This approach works when the application is running on > another core as it can replenish the fill ring while the driver is > busy-spinning. Though, this is a lousy approach if both of them are > running on the same core as the probability of the fill ring getting > more entries when the driver is busy-spinning is zero. With this new > feature the driver now sets the need_wakeup flag and returns to the > application. The application can then replenish the fill queue and > then explicitly wake up the Rx processing in the kernel using the > syscall poll(). For Tx, the flag is only set to one if the driver has > no outstanding Tx completion interrupts. If it has some, the flag is > zero as it will be woken up by a completion interrupt anyway. > > As a nice side effect, this new flag also improves the performance of > the case where application and driver are running on two different > cores as it reduces the number of syscalls to the kernel. The kernel > tells user space if it needs to be woken up by a syscall, and this > eliminates many of the syscalls. > > This flag needs some simple driver support. If the driver does not > support this, the Rx flag is always zero and the Tx flag is always > one. This makes any application relying on this feature default to the > old behaviour of not requiring any syscalls in the Rx path and always > having to call sendto() in the Tx path. > > For backwards compatibility reasons, this feature has to be explicitly > turned on using a new bind flag (XDP_USE_NEED_WAKEUP). I recommend > that you always turn it on as it so far always have had a positive > performance impact. > > The name and inspiration of the flag has been taken from io_uring by > Jens Axboe. Details about this feature in io_uring can be found in > http://kernel.dk/io_uring.pdf, section 8.3. > > Signed-off-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx> Acked-by: Jonathan Lemon <jonathan.lemon@xxxxxxxxx>
