netdev - Re: [PATCHv16 bpf-next 1/6] bpf: run devmap xdp_prog on flush instead of bulk enqueue

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Date:   Mon, 25 Jan 2021 11:30:25 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        Jiri Benc <jbenc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCHv16 bpf-next 1/6] bpf: run devmap xdp_prog on flush
 instead of bulk enqueue

On Fri, Jan 22, 2021 at 02:38:40PM +0100, Toke Høiland-Jørgensen wrote:
> >>  out:
> >> +	drops = cnt - sent;
> >>  	bq->count = 0;
> >>  
> >>  	trace_xdp_devmap_xmit(bq->dev_rx, dev, sent, drops, err);
> >>  	bq->dev_rx = NULL;
> >> +	bq->xdp_prog = NULL;
> >
> > One more question, do you really have to do that per each bq_xmit_all
> > call? Couldn't you clear it in __dev_flush ?
> >
> > Or IOW - what's the rationale behind storing xdp_prog in
> > xdp_dev_bulk_queue. Why can't you propagate the dst->xdp_prog and rely on
> > that without that local pointer?
> >
> > You probably have an answer for that, so maybe include it in commit
> > message.
> >
> > BTW same question for clearing dev_rx. To me this will be the same for all
> > bq_xmit_all() calls that will happen within same napi.
> 
> I think you're right: When bq_xmit_all() is called from bq_enqueue(),
> another packet will always be enqueued immediately after, so clearing
> out all of those things in bq_xmit_all() is redundant. This also
> includes the list_del on bq->flush_node, BTW.
> 
> And while we're getting into e micro-optimisations: In bq_enqueue() we
> have two checks:
> 
> 	if (!bq->dev_rx)
> 		bq->dev_rx = dev_rx;
> 
> 	bq->q[bq->count++] = xdpf;
> 
> 	if (!bq->flush_node.prev)
> 		list_add(&bq->flush_node, flush_list);
> 
> 
> those two if() checks can be collapsed into one, since the list and the
> dev_rx field are only ever modified together. This will also be the case
> for bq->xdp_prog, so putting all three under the same check in
> bq_enqueue() and only clearing them in __dev_flush() would be a win, I
> suppose - nice catch! :)

Thanks for the advice, so how about modify it like:

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index bc38f7193149..217e09533097 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -413,9 +413,6 @@ static void bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
 	bq->count = 0;
 
 	trace_xdp_devmap_xmit(bq->dev_rx, dev, sent, drops, err);
-	bq->dev_rx = NULL;
-	bq->xdp_prog = NULL;
-	__list_del_clearprev(&bq->flush_node);
 	return;
 }
 
@@ -434,8 +431,12 @@ void __dev_flush(void)
 	struct list_head *flush_list = this_cpu_ptr(&dev_flush_list);
 	struct xdp_dev_bulk_queue *bq, *tmp;
 
-	list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
+	list_for_each_entry_safe(bq, tmp, flush_list, flush_node) {
 		bq_xmit_all(bq, XDP_XMIT_FLUSH);
+		bq->dev_rx = NULL;
+		bq->xdp_prog = NULL;
+		__list_del_clearprev(&bq->flush_node);
+	}
 }
 
 /* rcu_read_lock (from syscall and BPF contexts) ensures that if a delete and/or
@@ -469,22 +470,17 @@ static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
 	/* Ingress dev_rx will be the same for all xdp_frame's in
 	 * bulk_queue, because bq stored per-CPU and must be flushed
 	 * from net_device drivers NAPI func end.
+	 *
+	 * Do the same with xdp_prog and flush_list since these fields
+	 * are modified together.
 	 */
-	if (!bq->dev_rx)
+	if (!bq->dev_rx) {
 		bq->dev_rx = dev_rx;
-
-	/* Store (potential) xdp_prog that run before egress to dev as
-	 * part of bulk_queue.  This will be same xdp_prog for all
-	 * xdp_frame's in bulk_queue, because this per-CPU store must
-	 * be flushed from net_device drivers NAPI func end.
-	 */
-	if (!bq->xdp_prog)
 		bq->xdp_prog = xdp_prog;
+		list_add(&bq->flush_node, flush_list);
+	}
 
 	bq->q[bq->count++] = xdpf;
-
-	if (!bq->flush_node.prev)
-		list_add(&bq->flush_node, flush_list);
 }
 
 static inline int __xdp_enqueue(struct net_device *dev, struct xdp_buff *xdp,