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

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Message-ID: <ed10eea2-0bf2-4747-b519-f9b9089e434e@huawei.com>
Date: Tue, 1 Apr 2025 15:43:54 +0800
From: Wang Liang <wangliang74@...wei.com>
To: Magnus Karlsson <magnus.karlsson@...il.com>
CC: Stanislav Fomichev <stfomichev@...il.com>, <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 14:57, Magnus Karlsson 写道:
> On Tue, 1 Apr 2025 at 04:36, Wang Liang <wangliang74@...wei.com> wrote:
>>
>> 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++;
> Sorry, but I do not understand how to reproduce this error. So you
> first issue a setsockopt with the XDP_TX_RING option and then you do
> not "reserve tx ring". What does that last "not reserve tx ring" mean?
> No mmap() of that ring, or something else? I guess you have bound the
> socket with a bind()? Some pseudo code on how to reproduce this would
> be helpful. Just want to understand so I can help. Thank you.
Sorry, the last email is garbled, and send again.

Ok. Some pseudo code like below:

     fd = socket(AF_XDP, SOCK_RAW, 0);
     setsockopt(fd, SOL_XDP, XDP_UMEM_REG, &mr, sizeof(mr));

     setsockopt(fd, SOL_XDP, XDP_UMEM_FILL_RING, &fill_size, 
sizeof(fill_size));
     setsockopt(fd, SOL_XDP, XDP_UMEM_COMPLETION_RING, &comp_size, 
sizeof(comp_size));
     mmap(NULL, off.fr.desc + fill_size * sizeof(__u64), ..., 
XDP_UMEM_PGOFF_FILL_RING);
     mmap(NULL, off.cr.desc + comp_size * sizeof(__u64), ..., 
XDP_UMEM_PGOFF_COMPLETION_RING);

     setsockopt(fd, SOL_XDP, XDP_RX_RING, &rx_size, sizeof(rx_size));
     setsockopt(fd, SOL_XDP, XDP_TX_RING, &tx_size, sizeof(tx_size));
     mmap(NULL, off.rx.desc + rx_size * sizeof(struct xdp_desc), ..., 
XDP_PGOFF_RX_RING);
     mmap(NULL, off.tx.desc + tx_size * sizeof(struct xdp_desc), ..., 
XDP_PGOFF_TX_RING);

     bind(fd, (struct sockaddr *)&sxdp, sizeof(sxdp));
     bpf_map_update_elem(xsk_map_fd, &queue_id, &fd, 0);

     while(!global_exit) {
         poll(fds, 1, -1);
         handle_receive_packets(...);
     }

The xsk is created success, and xs->tx is initialized.

The "not reserve tx ring" means user app do not update tx ring producer.
Like:

     xsk_ring_prod__reserve(tx, 1, &tx_idx);
     xsk_ring_prod__tx_desc(tx, tx_idx)->addr = frame;
     xsk_ring_prod__tx_desc(tx, tx_idx)->len = pkg_length;
     xsk_ring_prod__submit(tx, 1);

These functions (xsk_ring_prod__reserve, etc.) is provided by libxdp.

The tx->producer is not updated, so the xs->tx->cached_cons and
xs->tx->cached_prod are always zero.

When receive packets and user app call poll(), xsk_generic_xmit() will be
triggered by xsk_poll(), leading to this issue.
>>>>> 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);
>>           }
>>
>>