[kbuild-all] Re: [PATCH bpf-next v2 5/5] i40e: use batched xsk Tx interfaces to increase performance

On Wed, Nov 11, 2020 at 2:38 AM kernel test robot <lkp(a)intel.com&gt; wrote: > > Hi Magnus, > > I love your patch! Perhaps something to improve: > > [auto build test WARNING on bpf-next/master] > > url: https://github.com/0day-ci/linux/commits/Magnus-Karlsson/xsk-i40e-Tx-perf... > base: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master > config: powerpc64-randconfig-r025-20201110 (attached as .config) > compiler: clang version 12.0.0 (https://github.com/llvm/llvm-project 4d81c8adb6ed9840257f6cb6b93f60856d422a15)

> reproduce (this is a W=1 build): > wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross > chmod +x ~/bin/make.cross > # install powerpc64 cross compiling tool for clang build > # apt-get install binutils-powerpc64-linux-gnu > # https://github.com/0day-ci/linux/commit/b016bbeac6692a93e61b28efa430d6464... > git remote add linux-review https://github.com/0day-ci/linux > git fetch --no-tags linux-review Magnus-Karlsson/xsk-i40e-Tx-performance-improvements/20201110-190310 > git checkout b016bbeac6692a93e61b28efa430d64645032b5e > # save the attached .config to linux build tree > COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=powerpc64 > > If you fix the issue, kindly add following tag as appropriate > Reported-by: kernel test robot <lkp(a)intel.com&gt; > > All warnings (new ones prefixed by >>): > > >> drivers/net/ethernet/intel/i40e/i40e_xsk.c:417:13: warning: unknown pragma ignored [-Wunknown-pragmas] > #pragma GCC unroll 4 > ^ > 1 warning generated. And I was hoping that unknown pragmas would be ignored, but that will obviously not be the case with -Wunknown-pragmas added. The unrolling of this inner loop where the code spends most of its time gives me nearly 1 Mpps extra in performance which is substantial, so I would like to get this unrolled in some way, but without the warning. Need some advice please. Here are some options that comes in mind: #1: Suppress unknown pragma warnings in this file only by adding CFLAGS_i40e_xsk.o += -Wno-unknown-pragmas (or whatever that option might be) in the Makefile #2: Force the compiler to loop-unroll the loop with for example a switch statement with four cases that all fall through. This will make the code less readable. #3: Manually loop-unroll the loop. This will make the code even less readable than #2.

I prefer #1 as I like to keep the code readable, but you might have other better suggestions on how to tackle this. Thanks: Magnus > vim +417 drivers/net/ethernet/intel/i40e/i40e_xsk.c > > 408 > 409 static void i40e_xmit_pkt_batch(struct i40e_ring *xdp_ring, struct xdp_desc *desc, > 410 unsigned int *total_bytes) > 411 { > 412 u16 ntu = xdp_ring->next_to_use; > 413 struct i40e_tx_desc *tx_desc; > 414 dma_addr_t dma; > 415 u32 i; > 416 > > 417 #pragma GCC unroll 4 > 418 for (i = 0; i < PKTS_PER_BATCH; i++) { > 419 dma = xsk_buff_raw_get_dma(xdp_ring->xsk_pool, desc[i].addr); > 420 xsk_buff_raw_dma_sync_for_device(xdp_ring->xsk_pool, dma, desc[i].len); > 421 > 422 tx_desc = I40E_TX_DESC(xdp_ring, ntu++); > 423 tx_desc->buffer_addr = cpu_to_le64(dma); > 424 tx_desc->cmd_type_offset_bsz = build_ctob(I40E_TX_DESC_CMD_ICRC | > 425 I40E_TX_DESC_CMD_EOP, > 426 0, desc[i].len, 0); > 427 > 428 *total_bytes += desc[i].len; > 429 } > 430 > 431 xdp_ring->next_to_use = ntu; > 432 } > 433 > > --- > 0-DAY CI Kernel Test Service, Intel Corporation > https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org -- You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe(a)googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/CAJ8uoz2aDjLPtcTgZ_pO....