netdev - RE: [PATCH bpf-next v3 4/5] xsk: introduce batched Tx descriptor interfaces

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Message-ID: <5fb41f6ae195_310220813@john-XPS-13-9370.notmuch>
Date:   Tue, 17 Nov 2020 11:07:22 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>,
        magnus.karlsson@...el.com, bjorn.topel@...el.com, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        jonathan.lemon@...il.com, kuba@...nel.org, john.fastabend@...il.com
Cc:     bpf@...r.kernel.org, jeffrey.t.kirsher@...el.com,
        anthony.l.nguyen@...el.com, maciej.fijalkowski@...el.com,
        maciejromanfijalkowski@...il.com, intel-wired-lan@...ts.osuosl.org
Subject: RE: [PATCH bpf-next v3 4/5] xsk: introduce batched Tx descriptor
 interfaces

Magnus Karlsson wrote:
> From: Magnus Karlsson <magnus.karlsson@...el.com>
> 
> Introduce batched descriptor interfaces in the xsk core code for the
> Tx path to be used in the driver to write a code path with higher
> performance. This interface will be used by the i40e driver in the
> next patch. Though other drivers would likely benefit from this new
> interface too.
> 
> Note that batching is only implemented for the common case when
> there is only one socket bound to the same device and queue id. When
> this is not the case, we fall back to the old non-batched version of
> the function.
> 
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
> ---
>  include/net/xdp_sock_drv.h |  7 ++++
>  net/xdp/xsk.c              | 57 +++++++++++++++++++++++++++++
>  net/xdp/xsk_queue.h        | 89 +++++++++++++++++++++++++++++++++++++++-------
>  3 files changed, 140 insertions(+), 13 deletions(-)
> 

Acked-by: John Fastabend <john.fastabend@...il.com>

> +
> +u32 xsk_tx_peek_release_desc_batch(struct xsk_buff_pool *pool, struct xdp_desc *descs,
> +				   u32 max_entries)
> +{
> +	struct xdp_sock *xs;
> +	u32 nb_pkts;
> +
> +	rcu_read_lock();
> +	if (!list_is_singular(&pool->xsk_tx_list)) {
> +		/* Fallback to the non-batched version */

I'm going to ask even though I believe its correct.

If we fallback here and then an entry is added to the list while we are
in the fallback logic everything should still be OK, correct?

> +		rcu_read_unlock();
> +		return xsk_tx_peek_release_fallback(pool, descs, max_entries);
> +	}
> +
> +	xs = list_first_or_null_rcu(&pool->xsk_tx_list, struct xdp_sock, tx_list);
> +	if (!xs) {
> +		nb_pkts = 0;
> +		goto out;
> +	}
> +
> +	nb_pkts = xskq_cons_peek_desc_batch(xs->tx, descs, pool, max_entries);
> +	if (!nb_pkts) {
> +		xs->tx->queue_empty_descs++;
> +		goto out;
> +	}
> +
> +	/* This is the backpressure mechanism for the Tx path. Try to
> +	 * reserve space in the completion queue for all packets, but
> +	 * if there are fewer slots available, just process that many
> +	 * packets. This avoids having to implement any buffering in
> +	 * the Tx path.
> +	 */
> +	nb_pkts = xskq_prod_reserve_addr_batch(pool->cq, descs, nb_pkts);
> +	if (!nb_pkts)
> +		goto out;
> +
> +	xskq_cons_release_n(xs->tx, nb_pkts);
> +	__xskq_cons_release(xs->tx);
> +	xs->sk.sk_write_space(&xs->sk);
> +
> +out:
> +	rcu_read_unlock();
> +	return nb_pkts;
> +}
> +EXPORT_SYMBOL(xsk_tx_peek_release_desc_batch);
> +