linux-kernel - Re: [PATCH RFC net-next v2 7/7] net: skbuff: always try to recycle PP pages directly when in softirq

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Message-ID: <20230720110027.4bd43ee7@kernel.org>
Date:   Thu, 20 Jul 2023 11:00:27 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Larysa Zaremba <larysa.zaremba@...el.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Alexander Duyck <alexanderduyck@...com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        "Ilias Apalodimas" <ilias.apalodimas@...aro.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC net-next v2 7/7] net: skbuff: always try to recycle
 PP pages directly when in softirq

On Thu, 20 Jul 2023 19:48:06 +0200 Alexander Lobakin wrote:
> >> My question was "how can two things race on one CPU in one context if it
> >> implies they won't ever happen simultaneously", but maybe my zero
> >> knowledge of netcons hides something from me.  
> > 
> > One of them is in hardirq.  
> 
> If I got your message correctly, that means softirq_count() can return
> `true` even if we're in hardirq context, but there are some softirqs
> pending? 

Not pending, being executed. Hardirq can come during softirq.

> I.e. if I call local_irq_save() inside NAPI poll loop,
> in_softirq() will still return `true`? (I'll check it myself in a bit,
> but why not ask).

Yes.

> Isn't checking for `interrupt_context_level() == 1` more appropriate
> then? Page Pool core code also uses in_softirq(), as well as a hellaton
> of other networking-related places.

Right now page pool only supports BH and process contexts. IOW the
"else" branch of if (in_softirq()) in page pool is expecting to be
in process context.

Supporting hard irq would mean we need to switch to _irqsave() locking.
That's likely way too costly.

Or stash the freed pages away and free them lazily.

Or add a lockdep warning and hope nobody will ever free a page-pool
backed skb from hard IRQ context :)