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Jesper Dangaard Brouer says: ==================== This patchset change ndo_xdp_xmit API to take a bulk of xdp frames. When kernel is compiled with CONFIG_RETPOLINE, every indirect function pointer (branch) call hurts performance. For XDP this have a huge negative performance impact. This patchset reduce the needed (indirect) calls to ndo_xdp_xmit, but also prepares for further optimizations. The DMA APIs use of indirect function pointer calls is the primary source the regression. It is left for a followup patchset, to use bulking calls towards the DMA API (via the scatter-gatter calls). The other advantage of this API change is that drivers can easier amortize the cost of any sync/locking scheme, over the bulk of packets. The assumption of the current API is that the driver implemementing the NDO will also allocate a dedicated XDP TX queue for every CPU in the system. Which is not always possible or practical to configure. E.g. ixgbe cannot load an XDP program on a machine with more than 96 CPUs, due to limited hardware TX queues. E.g. virtio_net is hard to configure as it requires manually increasing the queues. E.g. tun driver chooses to use a per XDP frame producer lock modulo smp_processor_id over avail queues. I'm considered adding 'flags' to ndo_xdp_xmit, but it's not part of this patchset. This will be a followup patchset, once we know if this will be needed (e.g. for non-map xdp_redirect flush-flag, and if AF_XDP chooses to use ndo_xdp_xmit for TX). --- V5: Fixed up issues spotted by Daniel and John V4: Splitout the patches from 4 to 8 patches. I cannot split the driver changes from the NDO change, but I've tried to isolated the NDO change together with the driver change as much as possible. ==================== Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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.
See Documentation/00-INDEX for a list of what is contained in each file.
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.