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

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Date: Wed, 19 Jun 2024 10:05:41 +0200
From: Jiri Pirko <jiri@...nulli.us>
To: "Michael S. Tsirkin" <mst@...hat.com>
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

Wed, Jun 19, 2024 at 09:26:22AM CEST, mst@...hat.com wrote:
>On Wed, Jun 19, 2024 at 07:45:16AM +0200, Jiri Pirko wrote:
>> Tue, Jun 18, 2024 at 08:18:12PM CEST, mst@...hat.com 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 <jiri@...dia.com>
>> >> 
>> 
>> [...]
>> 
>> 
>> >> +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 ..

Well, the initialization happens in the caller by {0}, Wouldn't
memset in __free_old_xmit() add an extra overhead? IDK.
Perhaps a small comment in __free_old_xmit() would do better.

One way or another, I think this is parallel to this patchset. Will
handle it separatelly if you don't mind.

>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 ...
>> >
>> >
>> >>  }
>> >>  
>> 
>> [...]
>