Re: [PATCH RFC 1/2] virtio-net: bql support

[Jason Wang <jasowang@redhat.com>]

On 2018/12/26 下午11:22, Michael S. Tsirkin wrote:
> On Thu, Dec 06, 2018 at 04:17:36PM +0800, Jason Wang wrote:
>> On 2018/12/6 上午6:54, Michael S. Tsirkin wrote:
>>> When use_napi is set, let's enable BQLs.  Note: some of the issues are
>>> similar to wifi.  It's worth considering whether something similar to
>>> commit 36148c2bbfbe ("mac80211: Adjust TSQ pacing shift") might be
>>> benefitial.
>>
>> I've played a similar patch several days before. The tricky part is the mode
>> switching between napi and no napi. We should make sure when the packet is
>> sent and trakced by BQL,  it should be consumed by BQL as well.
>
> I just went over the patch again and I don't understand this comment.
> This patch only enabled BQL with tx napi.
>
> Thus there's no mode switching.
>
> What did I miss?


Consider the case:


TX NAPI is disabled:

send N packets

turn TX NAPI on:

get tx interrupt

BQL try to consume those packets when lead WARN for dql.


Thanks



>
>
>> I did it by
>> tracking it through skb->cb.  And deal with the freeze by reset the BQL
>> status. Patch attached.
>>
>> But when testing with vhost-net, I don't very a stable performance, it was
>> probably because we batch the used ring updating so tx interrupt may come
>> randomly. We probably need to implement time bounded coalescing mechanism
>> which could be configured from userspace.
>>
>> Btw, maybe it's time just enable napi TX by default. I get ~10% TCP_RR
>> regression on machine without APICv, (haven't found time to test APICv
>> machine). But consider it was for correctness, I think it's acceptable? Then
>> we can do optimization on top?
>>
>>
>> Thanks
>>
>>
>>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>>> ---
>>>    drivers/net/virtio_net.c | 27 +++++++++++++++++++--------
>>>    1 file changed, 19 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index cecfd77c9f3c..b657bde6b94b 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -1325,7 +1325,8 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
>>>    	return stats.packets;
>>>    }
>>> -static void free_old_xmit_skbs(struct send_queue *sq)
>>> +static void free_old_xmit_skbs(struct send_queue *sq, struct netdev_queue *txq,
>>> +			       bool use_napi)
>>>    {
>>>    	struct sk_buff *skb;
>>>    	unsigned int len;
>>> @@ -1347,6 +1348,9 @@ static void free_old_xmit_skbs(struct send_queue *sq)
>>>    	if (!packets)
>>>    		return;
>>> +	if (use_napi)
>>> +		netdev_tx_completed_queue(txq, packets, bytes);
>>> +
>>>    	u64_stats_update_begin(&sq->stats.syncp);
>>>    	sq->stats.bytes += bytes;
>>>    	sq->stats.packets += packets;
>>> @@ -1364,7 +1368,7 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
>>>    		return;
>>>    	if (__netif_tx_trylock(txq)) {
>>> -		free_old_xmit_skbs(sq);
>>> +		free_old_xmit_skbs(sq, txq, true);
>>>    		__netif_tx_unlock(txq);
>>>    	}
>>> @@ -1440,7 +1444,7 @@ static int virtnet_poll_tx(struct napi_struct *napi, int budget)
>>>    	struct netdev_queue *txq = netdev_get_tx_queue(vi->dev, vq2txq(sq->vq));
>>>    	__netif_tx_lock(txq, raw_smp_processor_id());
>>> -	free_old_xmit_skbs(sq);
>>> +	free_old_xmit_skbs(sq, txq, true);
>>>    	__netif_tx_unlock(txq);
>>>    	virtqueue_napi_complete(napi, sq->vq, 0);
>>> @@ -1505,13 +1509,15 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>>>    	struct send_queue *sq = &vi->sq[qnum];
>>>    	int err;
>>>    	struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
>>> -	bool kick = !skb->xmit_more;
>>> +	bool more = skb->xmit_more;
>>>    	bool use_napi = sq->napi.weight;
>>> +	unsigned int bytes = skb->len;
>>> +	bool kick;
>>>    	/* Free up any pending old buffers before queueing new ones. */
>>> -	free_old_xmit_skbs(sq);
>>> +	free_old_xmit_skbs(sq, txq, use_napi);
>>> -	if (use_napi && kick)
>>> +	if (use_napi && !more)
>>>    		virtqueue_enable_cb_delayed(sq->vq);
>>>    	/* timestamp packet in software */
>>> @@ -1552,7 +1558,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>>>    		if (!use_napi &&
>>>    		    unlikely(!virtqueue_enable_cb_delayed(sq->vq))) {
>>>    			/* More just got used, free them then recheck. */
>>> -			free_old_xmit_skbs(sq);
>>> +			free_old_xmit_skbs(sq, txq, false);
>>>    			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
>>>    				netif_start_subqueue(dev, qnum);
>>>    				virtqueue_disable_cb(sq->vq);
>>> @@ -1560,7 +1566,12 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>>>    		}
>>>    	}
>>> -	if (kick || netif_xmit_stopped(txq)) {
>>> +	if (use_napi)
>>> +		kick = __netdev_tx_sent_queue(txq, bytes, more);
>>> +	else
>>> +		kick = !more || netif_xmit_stopped(txq);
>>> +
>>> +	if (kick) {
>>>    		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
>>>    			u64_stats_update_begin(&sq->stats.syncp);
>>>    			sq->stats.kicks++;