On Mon, Mar 25, 2024 at 3:35 PM Robin Jarry <rja...@redhat.com> wrote:
>
> In some cases, the node context data is used to store two pointers
> because the data is larger than the reserved 16 bytes. Having to define
> intermediate structures just to be able to cast is tedious. Add two
> pointers that take the same space than ctx.
>
> Signed-off-by: Robin Jarry <rja...@redhat.com>
> ---
>
> Notes:
> v3:
>
> * Added __extension__ to the unnamed struct inside the union.
> * Fixed C++ header checks.
> * Replaced alignas() with an explicit static_assert.
>
> v2:
>
> * Added __extension__ (not sure where it is needed, I don't have access
> to windows).
> * It still fails the header check for C++. It seems not possible to align
> an unnamed union...
> Tyler, do you have an idea about how to fix that?
> * Added static_assert to ensure the anonymous union is not larger than
> RTE_NODE_CTX_SZ.
>
> lib/graph/rte_graph_worker_common.h | 21 ++++++++++++++++++++-
> 1 file changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/lib/graph/rte_graph_worker_common.h
> b/lib/graph/rte_graph_worker_common.h
> index 36d864e2c14e..722e9dac0d36 100644
> --- a/lib/graph/rte_graph_worker_common.h
> +++ b/lib/graph/rte_graph_worker_common.h
> @@ -12,7 +12,9 @@
> * process, enqueue and move streams of objects to the next nodes.
> */
>
> +#include <assert.h>
> #include <stdalign.h>
> +#include <stddef.h>
>
> #include <rte_common.h>
> #include <rte_cycles.h>
> @@ -112,7 +114,19 @@ struct __rte_cache_aligned rte_node {
> };
> /* Fast path area */
> #define RTE_NODE_CTX_SZ 16
> - alignas(RTE_CACHE_LINE_SIZE) uint8_t ctx[RTE_NODE_CTX_SZ]; /**< Node
> Context. */
> + /*
> + * alignas(RTE_CACHE_LINE_SIZE) cannot be used for ctx since it is
> part of an unnamed union.
> + * The compiler shifts the next field on the next cache line which is
> not what we want.
> + * The alignment is enforced via a explcicit static asserts below.
> + */
> + union {
> + uint8_t ctx[RTE_NODE_CTX_SZ];
> + /* Convenience aliases to store pointers without complex
> casting. */
> + __extension__ struct {
> + void *ctx_ptr;
> + void *ctx_ptr2;
> + };
> + }; /**< Node Context. */
> uint16_t size; /**< Total number of objects available. */
> uint16_t idx; /**< Number of objects used. */
> rte_graph_off_t off; /**< Offset of node in the graph reel. */
> @@ -130,6 +144,11 @@ struct __rte_cache_aligned rte_node {
> alignas(RTE_CACHE_LINE_MIN_SIZE) struct rte_node *nodes[]; /**< Next
> nodes. */
> };
>
> +static_assert(offsetof(struct rte_node, ctx) % RTE_CACHE_LINE_SIZE == 0,
> + "rte_node ctx must be aligned on a cache line");
This will fail in 32bit machine.
https://mails.dpdk.org/archives/test-report/2024-March/623806.html
I can think of following solution to add before ctx.
RTE_MARKER fastpath __rte_cache__aligned;
> +static_assert(offsetof(struct rte_node, size) - offsetof(struct rte_node,
> ctx) == RTE_NODE_CTX_SZ,
> + "rte_node context union cannot be larger than RTE_NODE_CTX_SZ");
> +
> /**
> * @internal
> *
> --
> 2.44.0
>
