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

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Date: Thu, 20 Jul 2023 20:01:28 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jakub Kicinski <kuba@...nel.org>
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

From: Jakub Kicinski <kuba@...nel.org>
Date: Thu, 20 Jul 2023 11:00:27 -0700

> 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 :)

I told you under the previous version that this function is not supposed
to be called under hardirq context, so we don't need to check for it :D
But I was assuming nobody would try to do that. Seems like not really
(netcons) if you want to sanitize this...

Thanks,
Olek