netdev - Re: [PATCH net-next v3 2/3] xsk: use atomic operations around cached

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Message-ID: <8fa70565-0f4a-4a73-a464-5530b2e29fa5@redhat.com>
Date: Fri, 28 Nov 2025 15:20:09 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Jason Xing <kerneljasonxing@...il.com>, davem@...emloft.net,
 edumazet@...gle.com, kuba@...nel.org, bjorn@...nel.org,
 magnus.karlsson@...el.com, maciej.fijalkowski@...el.com,
 jonathan.lemon@...il.com, sdf@...ichev.me, ast@...nel.org,
 daniel@...earbox.net, hawk@...nel.org, john.fastabend@...il.com,
 horms@...nel.org, andrew+netdev@...n.ch
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org,
 Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v3 2/3] xsk: use atomic operations around
 cached_prod for copy mode

On 11/28/25 2:46 PM, Jason Xing wrote:
> From: Jason Xing <kernelxing@...cent.com>
> 
> Use atomic_try_cmpxchg operations to replace spin lock. Technically
> CAS (Compare And Swap) is better than a coarse way like spin-lock
> especially when we only need to perform a few simple operations.
> Similar idea can also be found in the recent commit 100dfa74cad9
> ("net: dev_queue_xmit() llist adoption") that implements the lockless
> logic with the help of try_cmpxchg.
> 
> Signed-off-by: Jason Xing <kernelxing@...cent.com>
> ---
> Paolo, sorry that I didn't try to move the lock to struct xsk_queue
> because after investigation I reckon try_cmpxchg can add less overhead
> when multiple xsks contend at this point. So I hope this approach
> can be adopted.

I still think that moving the lock would be preferable, because it makes
sense also from a maintenance perspective. Can you report the difference
you measure atomics vs moving the spin lock?

Have you tried moving cq_prod_lock, too?

/P