On Wed, Nov 11, 2020 at 3:57 AM Magnus Karlsson <magnus.karlsson@xxxxxxxxx> wrote: > > On Wed, Nov 11, 2020 at 2:38 AM kernel test robot <lkp@xxxxxxxxx> 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-performance-improvements/20201110-190310 > > 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) ^ Note: clang > > 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/b016bbeac6692a93e61b28efa430d64645032b5e > > 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@xxxxxxxxx> > > > > 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. #4 support both compilers. Note Clang's syntax is slightly different here; it doesn't accept GCC specific pragmas, and uses a slightly different form: https://clang.llvm.org/docs/LanguageExtensions.html#loop-unrolling . If you wrap that in a macro based on `#ifdef __clang__`, that should do the trick. > > 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@xxxxxxxxxxxx > > -- > 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@xxxxxxxxxxxxxxxx. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/CAJ8uoz2aDjLPtcTgZ_pO-%3DS9TgXm3c57rN8TTPXdqT7HOOKrhA%40mail.gmail.com. -- Thanks, ~Nick Desaulniers
