netdev - Re: [EXT] Re: [PATCH net-next v2 3/4] octeon_ep: implement xmit

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Message-ID: <CANn89iKAn0-KBr00qrVibeM9Y7OqxDsg-keQQkty9oD1aCVtcA@mail.gmail.com>
Date: Thu, 26 Oct 2023 10:28:44 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Shinas Rasheed <srasheed@...vell.com>
Cc: Jakub Kicinski <kuba@...nel.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Haseeb Gani <hgani@...vell.com>, 
	Vimlesh Kumar <vimleshk@...vell.com>, "egallen@...hat.com" <egallen@...hat.com>, 
	"mschmidt@...hat.com" <mschmidt@...hat.com>, "pabeni@...hat.com" <pabeni@...hat.com>, 
	"horms@...nel.org" <horms@...nel.org>, "davem@...emloft.net" <davem@...emloft.net>, 
	"wizhao@...hat.com" <wizhao@...hat.com>, "konguyen@...hat.com" <konguyen@...hat.com>, 
	Veerasenareddy Burru <vburru@...vell.com>, Sathesh B Edara <sedara@...vell.com>
Subject: Re: [EXT] Re: [PATCH net-next v2 3/4] octeon_ep: implement xmit_more
 in transmit

On Thu, Oct 26, 2023 at 9:58 AM Shinas Rasheed <srasheed@...vell.com> wrote:
>
> Hi Jakub,
>
> Again, thanks for your review.
>
> > Does this guarantee that a full-sized skb can be accommodated?
> >> I assume by full-sized skb you mean a non-linear skb with MAX_SG_FRAGS elements in frags array.  Yes, this can be accommodated and the hardware uses separate SG list memory to siphon these skb frags instead of obtaining them from the same tx hardware queue. What I'm trying to say is, this means that a single tx descriptor will be enough for a full-sized skb as well, as hardware can pick up SG frags from separate memory and doesn't require separate descriptors.
>
> >If so - consider stopping stopping the queue when the condition is not true.
> >> We do stop the queue if tx queue is full, as in octep_iq_full_check earlier on.
>
> >The recommended way of implementing 'driver flow control'
> is to stop the queue once next packet may not fit, and then use
> netif_xmit_stopped() when deciding whether we need to flush or we can
> trust xmit_more. see
> https://urldefense.proofpoint.com/v2/url?u=https-3A__www.kernel.org_doc_html_next_networking_driver.html-23transmit-2Dpath-2Dguidelines&d=DwICAg&c=nKjWec2b6R0mOyPaz7xtfQ&r=1OxLD4y-oxrlgQ1rjXgWtmLz1pnaDjD96sDq-cKUwK4&m=FyJHTb5Z2u9DTFSYPU38S5kPcP5KvwGWzY-DPcqOl1gdnm7ToZhTFpyvhLMqh1hd&s=dBMmwfWKAi0UH3nrz7j9kYnAodDjuN3LZ5tC2aL_Prs&e=
>
> >> In the skeleton code above, as I understand each tx desc holds a skb frag and hence there can be possibility of receiving a full-sized skb, stopping the queue but on receiving another normal skb we should observe our queue to be stopped. This doesn't arise in our case as even if the skb is full-sized, it will only use a single tx descriptor so we can be sure if queue has been stopped, the write index will only be updated once posted (and read) tx descriptors are processed in napi context and queues awoken.
>
> Please correct me if I'm wrong anywhere (sorry if so) to further my understanding, and again thanks for your time!

Fact that octep_start_xmit() can return NETDEV_TX_BUSY is very suspicious.

I do not think a driver can implement xmit_more and keep
NETDEV_TX_BUSY at the same time.

Please make sure to remove NETDEV_TX_BUSY first, by stopping the queue earlier.