Re: [PATCH io_uring 02/16] io_uring/zcrx: move RQ head/tail to separate cache lines

On Fri, Aug 7, 2026 at 6:25 AM Pavel Begunkov <[email protected]> wrote:
>
> RQ head and tail are currently put into the same cache line, which can
> cause false sharing problems when refill is run on another CPU. Put them
> into separate cache lines.
>
> Signed-off-by: Pavel Begunkov <[email protected]>
> ---
>  io_uring/query.c | 2 +-
>  io_uring/zcrx.c  | 8 ++++----
>  io_uring/zcrx.h  | 7 ++++++-
>  3 files changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/io_uring/query.c b/io_uring/query.c
> index 88a325736992..4321f0b94b4b 100644
> --- a/io_uring/query.c
> +++ b/io_uring/query.c
> @@ -38,7 +38,7 @@ static ssize_t io_query_zcrx(union io_query_data *data)
>         e->register_flags = ZCRX_SUPPORTED_REG_FLAGS;
>         e->area_flags = IORING_ZCRX_AREA_DMABUF;
>         e->nr_ctrl_opcodes = __ZCRX_CTRL_LAST;
> -       e->rq_hdr_size = sizeof(struct io_uring);
> +       e->rq_hdr_size = sizeof(struct zcrx_rq_hdr);
>         e->rq_hdr_alignment = L1_CACHE_BYTES;
>         e->features = ZCRX_FEATURES;
>         e->__resv2 = 0;
> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
> index a096d44f7b52..fbc190075e2f 100644
> --- a/io_uring/zcrx.c
> +++ b/io_uring/zcrx.c
> @@ -379,9 +379,9 @@ static void io_zcrx_get_niov_uref(struct net_iov *niov)
>
>  static void io_fill_zcrx_offsets(struct io_uring_zcrx_offsets *offsets)
>  {
> -       offsets->head = offsetof(struct io_uring, head);
> -       offsets->tail = offsetof(struct io_uring, tail);
> -       offsets->rqes = ALIGN(sizeof(struct io_uring), L1_CACHE_BYTES);
> +       offsets->head = offsetof(struct zcrx_rq_hdr, head);
> +       offsets->tail = offsetof(struct zcrx_rq_hdr, tail);
> +       offsets->rqes = ALIGN(sizeof(struct zcrx_rq_hdr), L1_CACHE_BYTES);
>  }
>
>  static int io_allocate_rbuf_ring(struct io_ring_ctx *ctx,
> @@ -409,7 +409,7 @@ static int io_allocate_rbuf_ring(struct io_ring_ctx *ctx,
>                 return ret;
>
>         ptr = io_region_get_ptr(&ifq->rq_region);
> -       ifq->rq.ring = (struct io_uring *)ptr;
> +       ifq->rq.ring = (struct zcrx_rq_hdr *)ptr;
>         ifq->rq.rqes = (struct io_uring_zcrx_rqe *)(ptr + off);
>
>         memset(ifq->rq.ring, 0, sizeof(*ifq->rq.ring));
> diff --git a/io_uring/zcrx.h b/io_uring/zcrx.h
> index c1005f23caff..9ae0bf1c632f 100644
> --- a/io_uring/zcrx.h
> +++ b/io_uring/zcrx.h
> @@ -44,9 +44,14 @@ struct io_zcrx_area {
>         struct io_zcrx_mem      mem;
>  };
>
> +struct zcrx_rq_hdr {
> +       u32             head ____cacheline_aligned_in_smp;
> +       u32             tail ____cacheline_aligned_in_smp;
> +};
> +
>  struct zcrx_rq {
>         spinlock_t                      lock;
> -       struct io_uring                 *ring;
> +       struct zcrx_rq_hdr              *ring;

Is this not a problem in general for everything/most/a lot of using
struct io_uring? Should you make this change to struct io_uring
itself?

-- 
Thanks,
Mina