linux-kernel - Re: [PATCH net] xsk: correct tx_ring_empty

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Message-ID: <0d1b689c-c0ef-460a-9969-ff5aebbb8fac@huawei.com>
Date: Tue, 1 Apr 2025 10:35:45 +0800
From: Wang Liang <wangliang74@...wei.com>
To: Stanislav Fomichev <stfomichev@...il.com>
CC: <bjorn@...nel.org>, <magnus.karlsson@...el.com>,
	<maciej.fijalkowski@...el.com>, <jonathan.lemon@...il.com>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<pabeni@...hat.com>, <horms@...nel.org>, <ast@...nel.org>,
	<daniel@...earbox.net>, <hawk@...nel.org>, <john.fastabend@...il.com>,
	<yuehaibing@...wei.com>, <zhangchangzhong@...wei.com>,
	<netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] xsk: correct tx_ring_empty_descs count statistics


在 2025/4/1 6:03, Stanislav Fomichev 写道:
> On 03/31, Stanislav Fomichev wrote:
>> On 03/29, Wang Liang wrote:
>>> The tx_ring_empty_descs count may be incorrect, when set the XDP_TX_RING
>>> option but do not reserve tx ring. Because xsk_poll() try to wakeup the
>>> driver by calling xsk_generic_xmit() for non-zero-copy mode. So the
>>> tx_ring_empty_descs count increases once the xsk_poll()is called:
>>>
>>>    xsk_poll
>>>      xsk_generic_xmit
>>>        __xsk_generic_xmit
>>>          xskq_cons_peek_desc
>>>            xskq_cons_read_desc
>>>              q->queue_empty_descs++;
>>>
>>> To avoid this count error, add check for tx descs before send msg in poll.
>>>
>>> Fixes: df551058f7a3 ("xsk: Fix crash in poll when device does not support ndo_xsk_wakeup")
>>> Signed-off-by: Wang Liang <wangliang74@...wei.com>
>> Acked-by: Stanislav Fomichev <sdf@...ichev.me>
> Hmm, wait, I stumbled upon xskq_has_descs again and it looks only at
> cached prod/cons. How is it supposed to work when the actual tx
> descriptor is posted? Is there anything besides xskq_cons_peek_desc from
> __xsk_generic_xmit that refreshes cached_prod?


Yes, you are right!

How about using xskq_cons_nb_entries() to check free descriptors?

Like this:


diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index e5d104ce7b82..babb7928d335 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -993,7 +993,7 @@ static __poll_t xsk_poll(struct file *file, struct 
socket *sock,
         if (pool->cached_need_wakeup) {
                 if (xs->zc)
                         xsk_wakeup(xs, pool->cached_need_wakeup);
-               else if (xs->tx)
+               else if (xs->tx && xskq_cons_nb_entries(xs->tx, 1))
                         /* Poll needs to drive Tx also in copy mode */
                         xsk_generic_xmit(sk);
         }