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diff --git a/docs/reference/multiarch/nodefns.rst b/docs/reference/multiarch/nodefns.rst new file mode 100644 index 00000000000..ad68385d6c5 --- /dev/null +++ b/docs/reference/multiarch/nodefns.rst @@ -0,0 +1,160 @@ +Multi-Architecture Graph Node Cookbook +====================================== + +In the context of graph node dispatch functions, it's easy enough to +use the vpp multi-architecture support setup. The point of the scheme +is simple: for performance-critical nodes, generate multiple CPU +hardware-dependent versions of the node dispatch functions, and pick +the best one at runtime. + +The vpp scheme is simple enough to use, but details matter. + +100,000 foot view +----------------- + +We compile entire graph node dispatch function implementation files +multiple times. These compilations give rise to multiple versions of +the graph node dispatch functions. Per-node constructor-functions +interrogate CPU hardware, select the node dispatch function variant to +use, and set the vlib_node_registration_t ".function" member to the +address of the selected variant. + +Details +------- + +Declare the node dispatch function as shown, using the VLIB\_NODE\_FN macro. The +name of the node function **MUST** match the name of the graph node. + +:: + + VLIB_NODE_FN (ip4_sdp_node) (vlib_main_t * vm, vlib_node_runtime_t * node, + vlib_frame_t * frame) + { + if (PREDICT_FALSE (node->flags & VLIB_NODE_FLAG_TRACE)) + return ip46_sdp_inline (vm, node, frame, 1 /* is_ip4 */ , + 1 /* is_trace */ ); + else + return ip46_sdp_inline (vm, node, frame, 1 /* is_ip4 */ , + 0 /* is_trace */ ); + } + +We need to generate *precisely one copy* of the +vlib_node_registration_t, error strings, and packet trace decode function. + +Simply bracket these items with "#ifndef CLIB_MARCH_VARIANT...#endif": + +:: + + #ifndef CLIB_MARCH_VARIANT + static u8 * + format_sdp_trace (u8 * s, va_list * args) + { + <snip> + } + #endif + + ... + + #ifndef CLIB_MARCH_VARIANT + static char *sdp_error_strings[] = { + #define _(sym,string) string, + foreach_sdp_error + #undef _ + }; + #endif + + ... + + #ifndef CLIB_MARCH_VARIANT + VLIB_REGISTER_NODE (ip4_sdp_node) = + { + // DO NOT set the .function structure member. + // The multiarch selection __attribute__((constructor)) function + // takes care of it at runtime + .name = "ip4-sdp", + .vector_size = sizeof (u32), + .format_trace = format_sdp_trace, + .type = VLIB_NODE_TYPE_INTERNAL, + + .n_errors = ARRAY_LEN(sdp_error_strings), + .error_strings = sdp_error_strings, + + .n_next_nodes = SDP_N_NEXT, + + /* edit / add dispositions here */ + .next_nodes = + { + [SDP_NEXT_DROP] = "ip4-drop", + }, + }; + #endif + +To belabor the point: *do not* set the ".function" member! That's the job of the multi-arch +selection \_\_attribute\_\_((constructor)) function + +Always inline node dispatch functions +------------------------------------- + +It's typical for a graph dispatch function to contain one or more +calls to an inline function. See above. If your node dispatch function +is structured that way, make *ABSOLUTELY CERTAIN* to use the +"always_inline" macro: + +:: + + always_inline uword + ip46_sdp_inline (vlib_main_t * vm, vlib_node_runtime_t * node, + vlib_frame_t * frame, + int is_ip4, int is_trace) + { ... } + +Otherwise, the compiler is highly likely NOT to build multiple +versions of the guts of your dispatch function. + +It's fairly easy to spot this mistake in "perf top." If you see, for +example, a bunch of functions with names of the form +"xxx_node_fn_avx2" in the profile, *BUT* your brand-new node function +shows up with a name of the form "xxx_inline.isra.1", it's quite likely +that the inline was declared "static inline" instead of "always_inline". + +Add the required Makefile.am content +------------------------------------ + +If the component in question already sets a "multiversioning_sources" +variable, simply add the indicated .c file to the list. If not, add +the required boilerplate: + +:: + + if CPU_X86_64 + sdp_multiversioning_sources = \ + sdp/node.c \ + sdp/sdp_slookup.c + + if CC_SUPPORTS_AVX2 + ############################################################### + # AVX2 + ############################################################### + libsdp_plugin_avx2_la_SOURCES = $(sdp_multiversioning_sources) + libsdp_plugin_avx2_la_CFLAGS = \ + $(AM_CFLAGS) @CPU_AVX2_FLAGS@ \ + -DCLIB_MARCH_VARIANT=avx2 + noinst_LTLIBRARIES += libsdp_plugin_avx2.la + sdp_plugin_la_LIBADD += libsdp_plugin_avx2.la + endif + + if CC_SUPPORTS_AVX512 + ############################################################### + # AVX512 + ############################################################### + libsdp_plugin_avx512_la_SOURCES = $(sdp_multiversioning_sources) + libsdp_plugin_avx512_la_CFLAGS = \ + $(AM_CFLAGS) @CPU_AVX512_FLAGS@ \ + -DCLIB_MARCH_VARIANT=avx512 + noinst_LTLIBRARIES += libsdp_plugin_avx512.la + sdp_plugin_la_LIBADD += libsdp_plugin_avx512.la + endif + endif + +A certain amount of cut-paste-modify is currently required. Hopefully +we'll manage to improve the scheme in the future. |