netdev - Re: [PATCH net-next v3 05/18] xdp: Use nested-BH locking for system_page

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Message-ID: <20250502133231.lS281-FN@linutronix.de>
Date: Fri, 2 May 2025 15:32:31 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: netdev@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH net-next v3 05/18] xdp: Use nested-BH locking for
 system_page_pool

On 2025-05-01 12:13:24 [+0200], Toke Høiland-Jørgensen wrote:
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -462,7 +462,9 @@ EXPORT_PER_CPU_SYMBOL(softnet_data);
> >   * PP consumers must pay attention to run APIs in the appropriate context
> >   * (e.g. NAPI context).
> >   */
> > -DEFINE_PER_CPU(struct page_pool *, system_page_pool);
> > +DEFINE_PER_CPU(struct page_pool_bh, system_page_pool) = {
> > +	.bh_lock = INIT_LOCAL_LOCK(bh_lock),
> > +};
> 
> I'm a little fuzzy on how DEFINE_PER_CPU() works, but does this
> initialisation automatically do the right thing with the multiple
> per-CPU instances?

It sets the "first" per-CPU data which is then copied to all
"possible-CPUs" during early boot when the per-CPU data is made
available. You can initialize almost everything like that. Pointer based
structures (such as LIST_HEAD_INIT()) is something that obviously won't
work.

> >  #ifdef CONFIG_LOCKDEP
> >  /*
> > --- a/net/core/xdp.c
> > +++ b/net/core/xdp.c
> > @@ -737,10 +737,10 @@ static noinline bool xdp_copy_frags_from_zc(struct sk_buff *skb,
> >   */
> >  struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
> >  {
> > -	struct page_pool *pp = this_cpu_read(system_page_pool);
> >  	const struct xdp_rxq_info *rxq = xdp->rxq;
> >  	u32 len = xdp->data_end - xdp->data_meta;
> >  	u32 truesize = xdp->frame_sz;
> > +	struct page_pool *pp;
> >  	struct sk_buff *skb;
> >  	int metalen;
> >  	void *data;
> > @@ -748,13 +748,18 @@ struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
> >  	if (!IS_ENABLED(CONFIG_PAGE_POOL))
> >  		return NULL;
> >  
> > +	local_lock_nested_bh(&system_page_pool.bh_lock);
> > +	pp = this_cpu_read(system_page_pool.pool);
> >  	data = page_pool_dev_alloc_va(pp, &truesize);
> > -	if (unlikely(!data))
> > +	if (unlikely(!data)) {
> > +		local_unlock_nested_bh(&system_page_pool.bh_lock);
> >  		return NULL;
> > +	}
> >  
> >  	skb = napi_build_skb(data, truesize);
> >  	if (unlikely(!skb)) {
> >  		page_pool_free_va(pp, data, true);
> > +		local_unlock_nested_bh(&system_page_pool.bh_lock);
> >  		return NULL;
> >  	}
> >  
> > @@ -773,9 +778,11 @@ struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
> >  
> >  	if (unlikely(xdp_buff_has_frags(xdp)) &&
> >  	    unlikely(!xdp_copy_frags_from_zc(skb, xdp, pp))) {
> > +		local_unlock_nested_bh(&system_page_pool.bh_lock);
> >  		napi_consume_skb(skb, true);
> >  		return NULL;
> >  	}
> > +	local_unlock_nested_bh(&system_page_pool.bh_lock);
> 
> Hmm, instead of having four separate unlock calls in this function, how
> about initialising skb = NULL, and having the unlock call just above
> 'return skb' with an out: label?
> 
> Then the three topmost 'return NULL' can just straight-forwardly be
> replaced with 'goto out', while the last one becomes 'skb = NULL; goto
> out;'. I think that would be more readable than this repetition.

Something like the following maybe? We would keep the lock during
napi_consume_skb() which should work.

diff --git a/net/core/xdp.c b/net/core/xdp.c
index b2a5c934fe7b7..1ff0bc328305d 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -740,8 +740,8 @@ struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
 	const struct xdp_rxq_info *rxq = xdp->rxq;
 	u32 len = xdp->data_end - xdp->data_meta;
 	u32 truesize = xdp->frame_sz;
+	struct sk_buff *skb = NULL;
 	struct page_pool *pp;
-	struct sk_buff *skb;
 	int metalen;
 	void *data;
 
@@ -751,16 +751,13 @@ struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
 	local_lock_nested_bh(&system_page_pool.bh_lock);
 	pp = this_cpu_read(system_page_pool.pool);
 	data = page_pool_dev_alloc_va(pp, &truesize);
-	if (unlikely(!data)) {
-		local_unlock_nested_bh(&system_page_pool.bh_lock);
-		return NULL;
-	}
+	if (unlikely(!data))
+		goto out;
 
 	skb = napi_build_skb(data, truesize);
 	if (unlikely(!skb)) {
 		page_pool_free_va(pp, data, true);
-		local_unlock_nested_bh(&system_page_pool.bh_lock);
-		return NULL;
+		goto out;
 	}
 
 	skb_mark_for_recycle(skb);
@@ -778,15 +775,16 @@ struct sk_buff *xdp_build_skb_from_zc(struct xdp_buff *xdp)
 
 	if (unlikely(xdp_buff_has_frags(xdp)) &&
 	    unlikely(!xdp_copy_frags_from_zc(skb, xdp, pp))) {
-		local_unlock_nested_bh(&system_page_pool.bh_lock);
 		napi_consume_skb(skb, true);
-		return NULL;
+		skb = NULL;
 	}
+
+out:
 	local_unlock_nested_bh(&system_page_pool.bh_lock);
-
-	xsk_buff_free(xdp);
-
-	skb->protocol = eth_type_trans(skb, rxq->dev);
+	if (skb) {
+		xsk_buff_free(xdp);
+		skb->protocol = eth_type_trans(skb, rxq->dev);
+	}
 
 	return skb;
 }

> -Toke

Sebastian