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

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Message-ID: <20230720200700.162b29c6@kernel.org>
Date: Thu, 20 Jul 2023 20:07:00 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Wei Fang <wei.fang@....com>
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

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.