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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 19 Jun 2024 03:26:22 -0400

On Wed, Jun 19, 2024 at 07:45:16AM +0200, Jiri Pirko wrote:
> Tue, Jun 18, 2024 at 08:18:12PM CEST, [email-masked] wrote:
> >This looks like a sensible way to do this.
> >Yet something to improve:
> >
> >
> >On Tue, Jun 18, 2024 at 04:44:56PM +0200, Jiri Pirko wrote:
> >> From: Jiri Pirko <[email-masked]>
> >> 
> 
> [...]
> 
> 
> >> +static void __free_old_xmit(struct send_queue *sq, struct netdev_queue *txq,
> >> +			    bool in_napi, struct virtnet_sq_free_stats *stats)
> >>  {
> >>  	unsigned int len;
> >>  	void *ptr;
> >>  
> >>  	while ((ptr = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> >> -		++stats->packets;
> >> -
> >>  		if (!is_xdp_frame(ptr)) {
> >> -			struct sk_buff *skb = ptr;
> >> +			struct sk_buff *skb = ptr_to_skb(ptr);
> >>  
> >>  			pr_debug("Sent skb %p\n", skb);
> >>  
> >> -			stats->bytes += skb->len;
> >> +			if (is_orphan_skb(ptr)) {
> >> +				stats->packets++;
> >> +				stats->bytes += skb->len;
> >> +			} else {
> >> +				stats->napi_packets++;
> >> +				stats->napi_bytes += skb->len;
> >> +			}
> >>  			napi_consume_skb(skb, in_napi);
> >>  		} else {
> >>  			struct xdp_frame *frame = ptr_to_xdp(ptr);
> >>  
> >> +			stats->packets++;
> >>  			stats->bytes += xdp_get_frame_len(frame);
> >>  			xdp_return_frame(frame);
> >>  		}
> >>  	}
> >> +	netdev_tx_completed_queue(txq, stats->napi_packets, stats->napi_bytes);
> >
> >Are you sure it's right? You are completing larger and larger
> >number of bytes and packets each time.
> 
> Not sure I get you. __free_old_xmit() is always called with stats
> zeroed. So this is just sum-up of one queue completion run.
> I don't see how this could become "larger and larger number" as you
> describe.

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 ..
WDYT?

> 
> >
> >For example as won't this eventually trigger this inside dql_completed:
> >
> >        BUG_ON(count > num_queued - dql->num_completed);
> 
> Nope, I don't see how we can hit it. Do not complete anything else
> in addition to what was started in xmit(). Am I missing something?
> 
> 
> >
> >?
> >
> >
> >If I am right the perf testing has to be redone with this fixed ...
> >
> >
> >>  }
> >>  
> 
> [...]