On Wed, Nov 11, 2020 at 2:38 AM kernel test robot <[email protected]> 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)
> 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 <[email protected]>
>
> 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/[email protected]
