Subject: Re: [PATCH net-next v2 2/2] xsk: use a smaller new lock for shared pool case

From: Maciej Fijalkowski <[email-masked]>
Date: Mon, 3 Nov 2025 15:58:20 +0100

On Thu, Oct 30, 2025 at 08:06:46AM +0800, Jason Xing wrote:
> From: Jason Xing <[email-masked]>
> 
> - Split cq_lock into two smaller locks: cq_prod_lock and
>   cq_cached_prod_lock
> - Avoid disabling/enabling interrupts in the hot xmit path
> 
> In either xsk_cq_cancel_locked() or xsk_cq_reserve_locked() function,
> the race condition is only between multiple xsks sharing the same
> pool. They are all in the process context rather than interrupt context,
> so now the small lock named cq_cached_prod_lock can be used without
> handling interrupts.
> 
> While cq_cached_prod_lock ensures the exclusive modification of
> @cached_prod, cq_prod_lock in xsk_cq_submit_addr_locked() only cares
> about @producer and corresponding @desc. Both of them don't necessarily
> be consistent with @cached_prod protected by cq_cached_prod_lock.
> That's the reason why the previous big lock can be split into two
> smaller ones. Please note that SPSC rule is all about the global state
> of producer and consumer that can affect both layers instead of local
> or cached ones.
> 
> Frequently disabling and enabling interrupt are very time consuming
> in some cases, especially in a per-descriptor granularity, which now
> can be avoided after this optimization, even when the pool is shared by
> multiple xsks.
> 
> With this patch, the performance number[1] could go from 1,872,565 pps
> to 1,961,009 pps. It's a minor rise of around 5%.
> 
> [1]: taskset -c 1 ./xdpsock -i enp2s0f1 -q 0 -t -S -s 64
> 
> Signed-off-by: Jason Xing <[email-masked]>
> ---
>  include/net/xsk_buff_pool.h | 13 +++++++++----
>  net/xdp/xsk.c               | 15 ++++++---------
>  net/xdp/xsk_buff_pool.c     |  3 ++-
>  3 files changed, 17 insertions(+), 14 deletions(-)
> 
> diff --git a/include/net/xsk_buff_pool.h b/include/net/xsk_buff_pool.h
> index cac56e6b0869..92a2358c6ce3 100644
> --- a/include/net/xsk_buff_pool.h
> +++ b/include/net/xsk_buff_pool.h
> @@ -85,11 +85,16 @@ struct xsk_buff_pool {
>  	bool unaligned;
>  	bool tx_sw_csum;
>  	void *addrs;
> -	/* Mutual exclusion of the completion ring in the SKB mode. Two cases to protect:
> -	 * NAPI TX thread and sendmsg error paths in the SKB destructor callback and when
> -	 * sockets share a single cq when the same netdev and queue id is shared.
> +	/* Mutual exclusion of the completion ring in the SKB mode.
> +	 * Protect: NAPI TX thread and sendmsg error paths in the SKB
> +	 * destructor callback.
>  	 */
> -	spinlock_t cq_lock;
> +	spinlock_t cq_prod_lock;
> +	/* Mutual exclusion of the completion ring in the SKB mode.
> +	 * Protect: when sockets share a single cq when the same netdev
> +	 * and queue id is shared.
> +	 */
> +	spinlock_t cq_cached_prod_lock;

Nice that existing hole is utilized here.

Acked-by: Maciej Fijalkowski <[email-masked]>

>  	struct xdp_buff_xsk *free_heads[];
>  };
>  
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 7b0c68a70888..2f26c918d448 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -548,12 +548,11 @@ static int xsk_wakeup(struct xdp_sock *xs, u8 flags)
>  
>  static int xsk_cq_reserve_locked(struct xsk_buff_pool *pool)
>  {
> -	unsigned long flags;
>  	int ret;
>  
> -	spin_lock_irqsave(&pool->cq_lock, flags);
> +	spin_lock(&pool->cq_cached_prod_lock);
>  	ret = xskq_prod_reserve(pool->cq);
> -	spin_unlock_irqrestore(&pool->cq_lock, flags);
> +	spin_unlock(&pool->cq_cached_prod_lock);
>  
>  	return ret;
>  }
> @@ -566,7 +565,7 @@ static void xsk_cq_submit_addr_locked(struct xsk_buff_pool *pool,
>  	unsigned long flags;
>  	u32 idx;
>  
> -	spin_lock_irqsave(&pool->cq_lock, flags);
> +	spin_lock_irqsave(&pool->cq_prod_lock, flags);
>  	idx = xskq_get_prod(pool->cq);
>  
>  	xskq_prod_write_addr(pool->cq, idx,
> @@ -583,16 +582,14 @@ static void xsk_cq_submit_addr_locked(struct xsk_buff_pool *pool,
>  		}
>  	}
>  	xskq_prod_submit_n(pool->cq, descs_processed);
> -	spin_unlock_irqrestore(&pool->cq_lock, flags);
> +	spin_unlock_irqrestore(&pool->cq_prod_lock, flags);
>  }
>  
>  static void xsk_cq_cancel_locked(struct xsk_buff_pool *pool, u32 n)
>  {
> -	unsigned long flags;
> -
> -	spin_lock_irqsave(&pool->cq_lock, flags);
> +	spin_lock(&pool->cq_cached_prod_lock);
>  	xskq_prod_cancel_n(pool->cq, n);
> -	spin_unlock_irqrestore(&pool->cq_lock, flags);
> +	spin_unlock(&pool->cq_cached_prod_lock);
>  }
>  
>  static void xsk_inc_num_desc(struct sk_buff *skb)
> diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
> index 309075050b2a..00a4eddaa0cd 100644
> --- a/net/xdp/xsk_buff_pool.c
> +++ b/net/xdp/xsk_buff_pool.c
> @@ -90,7 +90,8 @@ struct xsk_buff_pool *xp_create_and_assign_umem(struct xdp_sock *xs,
>  	INIT_LIST_HEAD(&pool->xskb_list);
>  	INIT_LIST_HEAD(&pool->xsk_tx_list);
>  	spin_lock_init(&pool->xsk_tx_list_lock);
> -	spin_lock_init(&pool->cq_lock);
> +	spin_lock_init(&pool->cq_prod_lock);
> +	spin_lock_init(&pool->cq_cached_prod_lock);
>  	refcount_set(&pool->users, 1);
>  
>  	pool->fq = xs->fq_tmp;
> -- 
> 2.41.3
>