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With more NICs supporting CHECKSUM_COMPLETE, and IPv6 being widely used csum_partial() is heavily used with small amount of bytes, and is consuming many cycles. IPv6 header size, for instance, is 40 bytes. Another thing to consider is that NET_IP_ALIGN is 0 on x86, meaning that network headers are not word-aligned, unless the driver forces this. This means that csum_partial() fetches one u16 to 'align the buffer', then performs three u64 additions with carry in a loop, then a remaining u32, then a remaining u16. With this new version, it performs a loop only for the 64 bytes blocks, then the remaining is bisected. Testing on various CPUs, all of them show a big reduction in csum_partial() cost (by 50 to 80 %) Before: 4.16% [kernel] [k] csum_partial After: 0.83% [kernel] [k] csum_partial If run in a loop 1,000,000 times: Before: 26,922,913 cycles # 3846130.429 GHz 80,302,961 instructions # 2.98 insn per cycle 21,059,816 branches # 3008545142.857 M/sec 2,896 branch-misses # 0.01% of all branches After: 17,960,709 cycles # 3592141.800 GHz 41,292,805 instructions # 2.30 insn per cycle 11,058,119 branches # 2211623800.000 M/sec 2,997 branch-misses # 0.03% of all branches [ bp: Massage, merge in subsequent fixes into a single patch: - um compilation error due to missing load_unaligned_zeropad(): - Reported-by: kernel test robot <lkp@intel.com> - Link: https://lkml.kernel.org/r/20211118175239.1525650-1-eric.dumazet@gmail.com - Fix initial seed for odd buffers - Reported-by: Noah Goldstein <goldstein.w.n@gmail.com> - Link: https://lkml.kernel.org/r/20211125141817.3541501-1-eric.dumazet@gmail.com ] Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Borislav Petkov <bp@suse.de> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com> Link: https://lore.kernel.org/r/20211112161950.528886-1-eric.dumazet@gmail.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.