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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Date:   Tue, 18 Jul 2023 17:40:42 -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 Fri, 14 Jul 2023 19:08:52 +0200 Alexander Lobakin wrote:
> Suggested-by: Jakub Kicinski <kuba@...nel.org> # in_softirq()

I thought I said something along the lines as "if this is safe you can
as well" which falls short of a suggestion, cause I don't think it is
safe :)

> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index fc1470aab5cf..1c22fd33be6c 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -902,7 +902,7 @@ bool page_pool_return_skb_page(struct page *page, bool napi_safe)
>  	 * in the same context as the consumer would run, so there's
>  	 * no possible race.
>  	 */
> -	if (napi_safe) {
> +	if (napi_safe || in_softirq()) {
>  		const struct napi_struct *napi = READ_ONCE(pp->p.napi);
>  
>  		allow_direct = napi &&

What if we got here from netpoll? napi budget was 0, so napi_safe is
false, but in_softirq() can be true or false.

XDP SKB is a toy, I really don't think 3-4% in XDP SKB warrants the
risk here.