From: Daniel Borkmann Date: Sat, 17 Aug 2019 21:07:32 +0000 (+0200) Subject: Merge branch 'bpf-af-xdp-wakeup' X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=c8186c8064c4c234a3d2e3d60e25c1f55ec4f8b3;p=openwrt%2Fstaging%2Fblogic.git Merge branch 'bpf-af-xdp-wakeup' Magnus Karlsson says: ==================== This patch set adds support for a new flag called need_wakeup in the AF_XDP Tx and fill rings. When this flag is set by the driver, 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 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 to get from the fill ring. This approach works when the application and driver is running on different cores as the application 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. This flag can also be used in other situations where the driver needs to be woken up explicitly. As a nice side effect, this new flag also improves the Tx 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. The Rx performance of the 2-core case is on the other hand slightly worse, since there is a need to use a syscall now to wake up the driver, instead of the driver busy-spinning. It does waste less CPU cycles though, which might lead to better overall system performance. This new flag needs some simple driver support. If the driver does not support it, 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 behavior 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 has a large positive performance impact for the one core case and does not degrade 2 core performance and actually improves it for Tx heavy workloads. Here are some performance numbers measured on my local, non-performance optimized development system. That is why you are seeing numbers lower than the ones from Björn and Jesper. 64 byte packets at 40Gbit/s line rate. All results in Mpps. Cores == 1 means that both application and driver is executing on the same core. Cores == 2 that they are on different cores. Applications need_wakeup cores txpush rxdrop l2fwd --------------------------------------------------------------- n 1 0.07 0.06 0.03 y 1 21.6 8.2 6.5 n 2 32.3 11.7 8.7 y 2 33.1 11.7 8.7 Overall, the need_wakeup flag provides the same or better performance in all the micro-benchmarks. The reduction of sendto() calls in txpush is large. Only a few per second is needed. For l2fwd, the drop is 50% for the 1 core case and more than 99.9% for the 2 core case. Do not know why I am not seeing the same drop for the 1 core case yet. 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. It also addresses most of the denial of service and sendto() concerns raised by Maxim Mikityanskiy in https://www.spinics.net/lists/netdev/msg554657.html. The typical Tx part of an application will have to change from: ret = sendto(fd,....) to: if (xsk_ring_prod__needs_wakeup(&xsk->tx)) ret = sendto(fd,....) and th Rx part from: rcvd = xsk_ring_cons__peek(&xsk->rx, BATCH_SIZE, &idx_rx); if (!rcvd) return; to: rcvd = xsk_ring_cons__peek(&xsk->rx, BATCH_SIZE, &idx_rx); if (!rcvd) { if (xsk_ring_prod__needs_wakeup(&xsk->umem->fq)) ret = poll(fd,.....); return; } v3 -> v4: * Maxim found a possible race in the Tx part of the driver. The setting of the flag needs to happen before the sending, otherwise it might trigger this race. Fixed in ixgbe and i40e driver. * Mellanox support contributed by Maxim * Removed the XSK_DRV_CAN_SLEEP flag as it was not used anymore. Thanks to Sridhar for discovering this. * For consistency the feature is now always called need_wakeup. There were some places where it was referred to as might_sleep, but they have been removed. Thanks to Sridhar for spotting. * Fixed some typos in the commit messages v2 -> v3: * Converted the Mellanox driver to the new ndo in patch 1 as pointed out by Maxim * Fixed the compatibility code of XDP_MMAP_OFFSETS so it now works. v1 -> v2: * Fixed bisectability problem pointed out by Jakub * Added missing initiliztion of the Tx need_wakeup flag to 1 This patch has been applied against commit b753c5a7f99f ("Merge branch 'r8152-RX-improve'") Structure of the patch set: Patch 1: Replaces the ndo_xsk_async_xmit with ndo_xsk_wakeup to support waking up both Rx and Tx processing Patch 2: Implements the need_wakeup functionality in common code Patch 3-4: Add need_wakeup support to the i40e and ixgbe drivers Patch 5: Add need_wakeup support to libbpf Patch 6: Add need_wakeup support to the xdpsock sample application Patch 7-8: Add need_wakeup support to the Mellanox mlx5 driver ==================== Signed-off-by: Daniel Borkmann --- c8186c8064c4c234a3d2e3d60e25c1f55ec4f8b3