linux-kernel - Re: [PATCH bpf v2 2/2] xsk: change the tx writeable condition

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Date:   Fri, 27 Nov 2020 22:29:37 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, magnus.karlsson@...il.com
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andrii@...nel.org>,
        KP Singh <kpsingh@...omium.org>,
        "open list:XDP SOCKETS (AF_XDP)" <netdev@...r.kernel.org>,
        "open list:XDP SOCKETS (AF_XDP)" <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf v2 2/2] xsk: change the tx writeable condition

On 11/25/20 7:48 AM, Xuan Zhuo wrote:
> Modify the tx writeable condition from the queue is not full to the
> number of present tx queues is less than the half of the total number
> of queues. Because the tx queue not full is a very short time, this will
> cause a large number of EPOLLOUT events, and cause a large number of
> process wake up.
> 
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>

This one doesn't apply cleanly against bpf tree, please rebase. Small comment
inline while looking over the patch:

> ---
>   net/xdp/xsk.c       | 16 +++++++++++++---
>   net/xdp/xsk_queue.h |  6 ++++++
>   2 files changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 0df8651..22e35e9 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -211,6 +211,14 @@ static int __xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp, u32 len,
>   	return 0;
>   }
>   
> +static bool xsk_tx_writeable(struct xdp_sock *xs)
> +{
> +	if (xskq_cons_present_entries(xs->tx) > xs->tx->nentries / 2)
> +		return false;
> +
> +	return true;
> +}
> +
>   static bool xsk_is_bound(struct xdp_sock *xs)
>   {
>   	if (READ_ONCE(xs->state) == XSK_BOUND) {
> @@ -296,7 +304,8 @@ void xsk_tx_release(struct xsk_buff_pool *pool)
>   	rcu_read_lock();
>   	list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list) {
>   		__xskq_cons_release(xs->tx);
> -		xs->sk.sk_write_space(&xs->sk);
> +		if (xsk_tx_writeable(xs))
> +			xs->sk.sk_write_space(&xs->sk);
>   	}
>   	rcu_read_unlock();
>   }
> @@ -499,7 +508,8 @@ static int xsk_generic_xmit(struct sock *sk)
>   
>   out:
>   	if (sent_frame)
> -		sk->sk_write_space(sk);
> +		if (xsk_tx_writeable(xs))
> +			sk->sk_write_space(sk);
>   
>   	mutex_unlock(&xs->mutex);
>   	return err;
> @@ -556,7 +566,7 @@ static __poll_t xsk_poll(struct file *file, struct socket *sock,
>   
>   	if (xs->rx && !xskq_prod_is_empty(xs->rx))
>   		mask |= EPOLLIN | EPOLLRDNORM;
> -	if (xs->tx && !xskq_cons_is_full(xs->tx))
> +	if (xs->tx && xsk_tx_writeable(xs))
>   		mask |= EPOLLOUT | EPOLLWRNORM;
>   
>   	return mask;
> diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
> index b936c46..b655004 100644
> --- a/net/xdp/xsk_queue.h
> +++ b/net/xdp/xsk_queue.h
> @@ -307,6 +307,12 @@ static inline bool xskq_cons_is_full(struct xsk_queue *q)
>   		q->nentries;
>   }
>   
> +static inline __u64 xskq_cons_present_entries(struct xsk_queue *q)

Types prefixed with __ are mainly for user-space facing things like uapi headers,
so in-kernel should be u64. Is there a reason this is not done as u32 (and thus
same as producer and producer)?

> +{
> +	/* No barriers needed since data is not accessed */
> +	return READ_ONCE(q->ring->producer) - READ_ONCE(q->ring->consumer);
> +}
> +
>   /* Functions for producers */
>   
>   static inline u32 xskq_prod_nb_free(struct xsk_queue *q, u32 max)
>