Re: [PATCH v3] graph: expose node context as pointers

[Jerin Jacob <jerinjacobk@gmail.com>]

On Mon, Mar 25, 2024 at 3:35 PM Robin Jarry <rjarry@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 <rjarry@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
>