[xdp-hints] Re: [PATCH bpf-next v6 11/18] ice: put XDP meta sources assignment under a static key condition

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Thu, Oct 12, 2023 at 07:05:17PM +0200, Larysa Zaremba wrote:
> Usage of XDP hints requires putting additional information after the
> xdp_buff. In basic case, only the descriptor has to be copied on a
> per-packet basis, because xdp_buff permanently resides before per-ring
> metadata (cached time and VLAN protocol ID).
> 
> However, in ZC mode, xdp_buffs come from a pool, so memory after such
> buffer does not contain any reliable information, so everything has to be
> copied, damaging the performance.
> 
> Introduce a static key to enable meta sources assignment only when attached
> XDP program is device-bound.
> 
> This patch eliminates a 6% performance drop in ZC mode, which was a result
> of addition of XDP hints to the driver.
> 
> Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> ---
>  drivers/net/ethernet/intel/ice/ice.h      |  1 +
>  drivers/net/ethernet/intel/ice/ice_main.c | 14 ++++++++++++++
>  drivers/net/ethernet/intel/ice/ice_txrx.c |  3 ++-
>  drivers/net/ethernet/intel/ice/ice_xsk.c  |  3 +++
>  4 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/ice/ice.h b/drivers/net/ethernet/intel/ice/ice.h
> index 3d0f15f8b2b8..76d22be878a4 100644
> --- a/drivers/net/ethernet/intel/ice/ice.h
> +++ b/drivers/net/ethernet/intel/ice/ice.h
> @@ -210,6 +210,7 @@ enum ice_feature {
>  };
>  
>  DECLARE_STATIC_KEY_FALSE(ice_xdp_locking_key);
> +DECLARE_STATIC_KEY_FALSE(ice_xdp_meta_key);
>  
>  struct ice_channel {
>  	struct list_head list;
> diff --git a/drivers/net/ethernet/intel/ice/ice_main.c b/drivers/net/ethernet/intel/ice/ice_main.c
> index 47e8920e1727..ee0df86d34b7 100644
> --- a/drivers/net/ethernet/intel/ice/ice_main.c
> +++ b/drivers/net/ethernet/intel/ice/ice_main.c
> @@ -48,6 +48,9 @@ MODULE_PARM_DESC(debug, "netif level (0=none,...,16=all)");
>  DEFINE_STATIC_KEY_FALSE(ice_xdp_locking_key);
>  EXPORT_SYMBOL(ice_xdp_locking_key);
>  
> +DEFINE_STATIC_KEY_FALSE(ice_xdp_meta_key);
> +EXPORT_SYMBOL(ice_xdp_meta_key);
> +
>  /**
>   * ice_hw_to_dev - Get device pointer from the hardware structure
>   * @hw: pointer to the device HW structure
> @@ -2634,6 +2637,11 @@ static int ice_xdp_alloc_setup_rings(struct ice_vsi *vsi)
>  	return -ENOMEM;
>  }
>  
> +static bool ice_xdp_prog_has_meta(struct bpf_prog *prog)
> +{
> +	return prog && prog->aux->dev_bound;
> +}
> +
>  /**
>   * ice_vsi_assign_bpf_prog - set or clear bpf prog pointer on VSI
>   * @vsi: VSI to set the bpf prog on
> @@ -2644,10 +2652,16 @@ static void ice_vsi_assign_bpf_prog(struct ice_vsi *vsi, struct bpf_prog *prog)
>  	struct bpf_prog *old_prog;
>  	int i;
>  
> +	if (ice_xdp_prog_has_meta(prog))
> +		static_branch_inc(&ice_xdp_meta_key);

i thought boolean key would be enough but inc/dec should serve properly
for example prog hotswap cases.

> +
>  	old_prog = xchg(&vsi->xdp_prog, prog);
>  	ice_for_each_rxq(vsi, i)
>  		WRITE_ONCE(vsi->rx_rings[i]->xdp_prog, vsi->xdp_prog);
>  
> +	if (ice_xdp_prog_has_meta(old_prog))
> +		static_branch_dec(&ice_xdp_meta_key);
> +
>  	if (old_prog)
>  		bpf_prog_put(old_prog);
>  }
> diff --git a/drivers/net/ethernet/intel/ice/ice_txrx.c b/drivers/net/ethernet/intel/ice/ice_txrx.c
> index 4fd7614f243d..19fc182d1f4c 100644
> --- a/drivers/net/ethernet/intel/ice/ice_txrx.c
> +++ b/drivers/net/ethernet/intel/ice/ice_txrx.c
> @@ -572,7 +572,8 @@ ice_run_xdp(struct ice_rx_ring *rx_ring, struct xdp_buff *xdp,
>  	if (!xdp_prog)
>  		goto exit;
>  
> -	ice_xdp_meta_set_desc(xdp, eop_desc);
> +	if (static_branch_unlikely(&ice_xdp_meta_key))

My only concern is that we might be hurting in a minor way hints path now,
no?

> +		ice_xdp_meta_set_desc(xdp, eop_desc);
>  
>  	act = bpf_prog_run_xdp(xdp_prog, xdp);
>  	switch (act) {
> diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c b/drivers/net/ethernet/intel/ice/ice_xsk.c
> index 39775bb6cec1..f92d7d33fde6 100644
> --- a/drivers/net/ethernet/intel/ice/ice_xsk.c
> +++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
> @@ -773,6 +773,9 @@ static void ice_prepare_pkt_ctx_zc(struct xdp_buff *xdp,
>  				   union ice_32b_rx_flex_desc *eop_desc,
>  				   struct ice_rx_ring *rx_ring)
>  {
> +	if (!static_branch_unlikely(&ice_xdp_meta_key))
> +		return;

wouldn't it be better to pull it out and avoid calling
ice_prepare_pkt_ctx_zc() unnecessarily?

> +
>  	XSK_CHECK_PRIV_TYPE(struct ice_xdp_buff);
>  	((struct ice_xdp_buff *)xdp)->pkt_ctx = rx_ring->pkt_ctx;
>  	ice_xdp_meta_set_desc(xdp, eop_desc);
> -- 
> 2.41.0
>