+++ /dev/null
-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.
Code Review / vpp.git / blobdiff