netdev - RE: [PATCH net] docs: net: clarify the NAPI rules around XDP Tx

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Date: Fri, 21 Jul 2023 04:31:46 +0000
From: Wei Fang <wei.fang@....com>
To: Jakub Kicinski <kuba@...nel.org>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "edumazet@...gle.com"
	<edumazet@...gle.com>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"corbet@....net" <corbet@....net>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCH net] docs: net: clarify the NAPI rules around XDP Tx

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: 2023年7月21日 11:07
> To: Wei Fang <wei.fang@....com>
> Cc: netdev@...r.kernel.org; edumazet@...gle.com; pabeni@...hat.com;
> corbet@....net; linux-doc@...r.kernel.org; davem@...emloft.net
> Subject: Re: [PATCH net] docs: net: clarify the NAPI rules around XDP Tx
> 
> On Fri, 21 Jul 2023 02:35:41 +0000 Wei Fang wrote:
> > > -In other words, it is recommended to ignore the budget argument
> > > when -performing TX buffer reclamation to ensure that the
> > > reclamation is not -arbitrarily bounded; however, it is required to
> > > honor the budget argument -for RX processing.
> > > +In other words for Rx processing the ``budget`` argument limits how
> > > +many packets driver can process in a single poll. Rx specific APIs
> > > +like page pool or XDP cannot be used at all when ``budget`` is 0.
> > > +skb Tx processing should happen regardless of the ``budget``, but
> > > +if the argument is 0 driver cannot call any XDP (or page pool) APIs.
> > >
> > Can I ask a stupid question why tx processing cannot call any XDP (or
> > page pool) APIs if the "budget" is 0?
> 
> Because in that case we may be in an interrupt context, and page pool
> assumes it's either in process or softirq context. See commit
> afbed3f74830 ("net/mlx5e: do as little as possible in napi poll when budget is
> 0") for an example stack trace.
I got it, thank you!