Message-ID: <16797fe9-2080-4e1e-ba2c-aa8e13febe8f@huawei.com>
Date: Wed, 2 Apr 2025 10:38:55 +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 19:00, Magnus Karlsson 写道:
> On Tue, 1 Apr 2025 at 11:33, Wang Liang <wangliang74@...wei.com> wrote:
>>
>> 在 2025/4/1 16:12, Magnus Karlsson 写道:
>>> On Tue, 1 Apr 2025 at 09:44, Wang Liang <wangliang74@...wei.com> wrote:
>>>> 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.
>>> Thanks, that really helped. The problem here is that the kernel cannot
>>> guess your intent. Since you created a socket with both Rx and Tx, it
>>> thinks you will use it for both, so it should increase
>>> queue_empty_descs in this case as you did not provide any Tx descs.
>>> Your proposed patch will break this. Consider this Tx case with the
>>> exact same init code as you have above but with this send loop:
>>>
>>> while(!global_exit) {
>>>          maybe_send_packets(...);
>>>          poll(fds, 1, -1);
>>> }
>>>
>>> With your patch, the queue_empty_descs will never be increased in the
>>> case when I do not submit any Tx descs, even though we would like it
>>> to be so.
>>>
>>> So in my mind, you have a couple of options:
>>>
>>> * Create two sockets, one rx only and one tx only and use the
>>> SHARED_UMEM mode to bind them to the same netdev and queue id. In your
>>> loop above, you would use the Rx socket. This might have the drawback
>>> that you need to call poll() twice if you are both sending and
>>> receiving in the same loop. But the stats will be the way you want
>>> them to be.
>>>
>>> * Introduce a new variable in user space that you increase every time
>>> you do poll() in your loop above. When displaying the statistics, just
>>> deduct this variable from the queue_empty_descs that the kernel
>>> reports using the XDP_STATISTICS getsockopt().
>>>
>>> Hope this helps.
>>
>> Thank you for the advices.
>>
>>   From user view, queue_empty_descs increases when the app only receive
>> packets and call poll(), it is some confusing.
> But if the only thing you are doing in the app is receive packets, you
> should create an Rx only socket.
>
>> In your Tx case, if user app use sendto() to send packets, the
>> queue_empty_descs will increase. This is reasonably.
>>
>> In linux manual, poll() waits for some event on a file descriptor. But
>> in af_xdp, poll() has a side effect of send msg.
> If I remember correctly, we did this so that zero-copy mode and copy
> mode should have the same behavior from an app point of view. But I do
> not remember why we implemented this behavior in zero-copy in the
> first place. Maybe out of necessity since zero-copy Tx is driven by
> the driver.
>
>> Previous commit 77cd0d7b3f25 ("xsk: add support for need_wakeup flag in
>> AF_XDP rings") add need_wakeup flag. It mainly work in rx process. When
>> the application and driver run on the same core, if the fill ring is
>> empty, the driver can set the need_wakeup flag and return, so the
>> application could be scheduled to produce entries of the fill ring.
>>
>> The commit df551058f7a3 ("xsk: Fix crash in poll when device does not
>> support ndo_xsk_wakeup") add sendmsg function in xsk_poll(), considering
>> some devices donot define ndo_xsk_wakeup.
>>
>> At present the value of need_wakeup & XDP_WAKEUP_TX is always true,
>> except the mlx5 driver. If the mlx5 driver queued some packets for tx,
>> it may clear XDP_WAKEUP_TX by xsk_clear_tx_need_wakeup(). So when user
>> app use sendto() to send packets, __xsk_sendmsg() will return without
>> calling xsk_generic_xmit().
> Mellanox is doing it in the correct way. The Intel drivers had it like
> this too in the beginning, until we discovered a race condition in the
> HW in which the interrupt was not armed at the same time as the
> need_wakeup flag was cleared. This would then lead to packets not
> being sent and user-space not knowing about it. As to why all other
> drivers copied this unfortunate fall-back, I do not know.
>
>> So I have a bold suggestion, how about removing xsk_generic_xmit() in
>> xsk_poll()?
> That would indeed be bold :-)! But we cannot do this since it would
> break existing applications that rely on this. I think you have to use
> one of the workarounds I pitched in the previous mail. If you are
> really just receiving, you should create an Rx only socket.
Thanks for your replies. it is really helpful.
>>>>>>>>> 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);
>>>>>>             }
>>>>>>
>>>>>>

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