netdev - Re: [PATCH net-next v3] virtio_net: add support for Byte Queue Limits

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Date: Thu, 20 Jun 2024 03:43:24 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jiri Pirko <jiri@...nulli.us>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, jasowang@...hat.com,
	xuanzhuo@...ux.alibaba.com, virtualization@...ts.linux.dev,
	ast@...nel.org, daniel@...earbox.net, hawk@...nel.org,
	john.fastabend@...il.com, dave.taht@...il.com,
	kerneljasonxing@...il.com, hengqi@...ux.alibaba.com
Subject: Re: [PATCH net-next v3] virtio_net: add support for Byte Queue Limits

On Wed, Jun 19, 2024 at 12:09:45PM +0200, Jiri Pirko wrote:
> Wed, Jun 19, 2024 at 10:23:25AM CEST, mst@...hat.com wrote:
> >On Wed, Jun 19, 2024 at 10:05:41AM +0200, Jiri Pirko wrote:
> >> >Oh. Right of course. Worth a comment maybe? Just to make sure
> >> >we remember not to call __free_old_xmit twice in a row
> >> >without reinitializing stats.
> >> >Or move the initialization into __free_old_xmit to make it
> >> >self-contained ..
> >> 
> >> Well, the initialization happens in the caller by {0}, Wouldn't
> >> memset in __free_old_xmit() add an extra overhead? IDK.
> >
> >
> >Well if I did the below the binary is a bit smaller.
> >
> >If you have to respin you can include it.
> >If not I can submit separately.
> 
> Please send it reparately. It's should be a separate patch.
> 
> Thanks!
> 
> >
> >----
> >
> >
> >virtio-net: cleanup __free_old_xmit
> >
> >Two call sites of __free_old_xmit zero-initialize stats,
> >doing it inside __free_old_xmit seems to make compiler's
> >job a bit easier:
> >
> >$ size /tmp/orig/virtio_net.o 
> >   text    data     bss     dec     hex filename
> >  65857    3892     100   69849   110d9 /tmp/orig/virtio_net.o
> >$ size /tmp/new/virtio_net.o 
> >   text    data     bss     dec     hex filename
> >  65760    3892     100   69752   11078 /tmp/new/virtio_net.o
> >
> >Couldn't measure any performance impact, unsurprizingly.
> >
> >Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> >
> >---
> >
> >diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >index 283b34d50296..c2ce8de340f7 100644
> >--- a/drivers/net/virtio_net.c
> >+++ b/drivers/net/virtio_net.c
> >@@ -383,6 +383,8 @@ static void __free_old_xmit(struct send_queue *sq, bool in_napi,
> > 	unsigned int len;
> > 	void *ptr;
> > 
> >+	stats->bytes = stats->packets = 0;
> 
> Memset perhaps?

Generates the same code and I find it less readable -
virtio generally opts for explicit initialization.

> >+
> > 	while ((ptr = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> > 		++stats->packets;
> > 
> >@@ -828,7 +830,7 @@ static void virtnet_rq_unmap_free_buf(struct virtqueue *vq, void *buf)
> > 
> > static void free_old_xmit(struct send_queue *sq, bool in_napi)
> > {
> >-	struct virtnet_sq_free_stats stats = {0};
> >+	struct virtnet_sq_free_stats stats;
> > 
> > 	__free_old_xmit(sq, in_napi, &stats);
> > 
> >@@ -979,7 +981,7 @@ static int virtnet_xdp_xmit(struct net_device *dev,
> > 			    int n, struct xdp_frame **frames, u32 flags)
> > {
> > 	struct virtnet_info *vi = netdev_priv(dev);
> >-	struct virtnet_sq_free_stats stats = {0};
> >+	struct virtnet_sq_free_stats stats;
> > 	struct receive_queue *rq = vi->rq;
> > 	struct bpf_prog *xdp_prog;
> > 	struct send_queue *sq;
> >