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Date:	Mon, 21 Jul 2014 11:13:05 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Vlad Yasevich <vyasevic@...hat.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next V2 3/3] virtio-net: rx busy polling support

On 07/21/2014 04:31 AM, Michael S. Tsirkin wrote:
> On Wed, Jul 16, 2014 at 02:21:47PM +0800, Jason Wang wrote:
>> Add basic support for rx busy polling.
>>
>> Test was done between a kvm guest and an external host. Two hosts were
>> connected through 40gb mlx4 cards. With both busy_poll and busy_read
>> are set to 50 in guest, 1 byte netperf tcp_rr shows 116% improvement:
>> transaction rate was increased from 9151.94 to 19787.37.
> Pls include data about non polling tests: any effect on
> cpu utilization there?
> There could be as we are adding locking.

I will do some test on this.
>
>> Cc: Rusty Russell <rusty@...tcorp.com.au>
>> Cc: Michael S. Tsirkin <mst@...hat.com>
>> Cc: Vlad Yasevich <vyasevic@...hat.com>
>> Cc: Eric Dumazet <eric.dumazet@...il.com>
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>  drivers/net/virtio_net.c | 190 ++++++++++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 187 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index e417d93..4830713 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -27,6 +27,7 @@
>>  #include <linux/slab.h>
>>  #include <linux/cpu.h>
>>  #include <linux/average.h>
>> +#include <net/busy_poll.h>
>>  
>>  static int napi_weight = NAPI_POLL_WEIGHT;
>>  module_param(napi_weight, int, 0444);
>> @@ -94,8 +95,143 @@ struct receive_queue {
>>  
>>  	/* Name of this receive queue: input.$index */
>>  	char name[40];
>> +
>> +#ifdef CONFIG_NET_RX_BUSY_POLL
>> +	unsigned int state;
>> +#define VIRTNET_RQ_STATE_IDLE        0
>> +#define VIRTNET_RQ_STATE_NAPI	     1    /* NAPI or refill owns this RQ */
>> +#define VIRTNET_RQ_STATE_POLL	     2    /* poll owns this RQ */
>> +#define VIRTNET_RQ_STATE_DISABLED    4    /* RQ is disabled */
>> +#define VIRTNET_RQ_OWNED (VIRTNET_RQ_STATE_NAPI | VIRTNET_RQ_STATE_POLL)
>> +#define VIRTNET_RQ_LOCKED (VIRTNET_RQ_OWNED | VIRTNET_RQ_STATE_DISABLED)
>> +#define VIRTNET_RQ_STATE_NAPI_YIELD  8    /* NAPI or refill yielded this RQ */
>> +#define VIRTNET_RQ_STATE_POLL_YIELD  16   /* poll yielded this RQ */
>> +	spinlock_t lock;
>> +#endif  /* CONFIG_NET_RX_BUSY_POLL */
> do we have to have a new state? no way to reuse the napi state
> for this? two lock/unlock operations for a poll seems
> excessive.

I try this way and it works. The only usage I can think of introducing
those states is to detect the yield and do some optimizations after. But
only few drivers (bnx2x) use the yield flag.

I think I can  switch to use NAPI state since we don't do such
optimization in virtio-net.
>
>>  };
>>  
>> +#ifdef CONFIG_NET_RX_BUSY_POLL
>> +static inline void virtnet_rq_init_lock(struct receive_queue *rq)
>> +{
>> +
>> +	spin_lock_init(&rq->lock);
>> +	rq->state = VIRTNET_RQ_STATE_IDLE;
>> +}
>> +
>> +/* called from the device poll routine or refill routine to get ownership of a
>> + * receive queue.
>> + */
>> +static inline bool virtnet_rq_lock_napi_refill(struct receive_queue *rq)
>> +{
>> +	int rc = true;
>> +
>> +	spin_lock(&rq->lock);
>> +	if (rq->state & VIRTNET_RQ_LOCKED) {
>> +		WARN_ON(rq->state & VIRTNET_RQ_STATE_NAPI);
>> +		rq->state |= VIRTNET_RQ_STATE_NAPI_YIELD;
>> +		rc = false;
>> +	} else
>> +		/* we don't care if someone yielded */
>> +		rq->state = VIRTNET_RQ_STATE_NAPI;
>> +	spin_unlock(&rq->lock);
>> +	return rc;
>> +}
>> +
>> +/* returns true is someone tried to get the rq while napi or refill had it */
> s/is/if/
>
>> +static inline bool virtnet_rq_unlock_napi_refill(struct receive_queue *rq)
>> +{
>> +	int rc = false;
>> +
>> +	spin_lock(&rq->lock);
>> +	WARN_ON(rq->state & (VIRTNET_RQ_STATE_POLL |
>> +			     VIRTNET_RQ_STATE_NAPI_YIELD));
>> +
>> +	if (rq->state & VIRTNET_RQ_STATE_POLL_YIELD)
>> +		rc = true;
>> +	/* will reset state to idle, unless RQ is disabled */
>> +	rq->state &= VIRTNET_RQ_STATE_DISABLED;
>> +	spin_unlock(&rq->lock);
>> +	return rc;
>> +}
>> +
>> +/* called from virtnet_low_latency_recv() */
>> +static inline bool virtnet_rq_lock_poll(struct receive_queue *rq)
>> +{
>> +	int rc = true;
>> +
>> +	spin_lock_bh(&rq->lock);
>> +	if ((rq->state & VIRTNET_RQ_LOCKED)) {
>> +		rq->state |= VIRTNET_RQ_STATE_POLL_YIELD;
>> +		rc = false;
>> +	} else
>> +		/* preserve yield marks */
>> +		rq->state |= VIRTNET_RQ_STATE_POLL;
>> +	spin_unlock_bh(&rq->lock);
>> +	return rc;
>> +}
>> +
>> +/* returns true if someone tried to get the receive queue while it was locked */
>> +static inline bool virtnet_rq_unlock_poll(struct receive_queue *rq)
>> +{
>> +	int rc = false;
>> +
>> +	spin_lock_bh(&rq->lock);
>> +	WARN_ON(rq->state & (VIRTNET_RQ_STATE_NAPI));
>> +
>> +	if (rq->state & VIRTNET_RQ_STATE_POLL_YIELD)
>> +		rc = true;
>> +	/* will reset state to idle, unless RQ is disabled */
>> +	rq->state &= VIRTNET_RQ_STATE_DISABLED;
>> +	spin_unlock_bh(&rq->lock);
>> +	return rc;
>> +}
>> +
>> +/* return false if RQ is currently owned */
>> +static inline bool virtnet_rq_disable(struct receive_queue *rq)
>> +{
>> +	int rc = true;
>> +
>> +	spin_lock_bh(&rq->lock);
>> +	if (rq->state & VIRTNET_RQ_OWNED)
>> +		rc = false;
>> +	rq->state |= VIRTNET_RQ_STATE_DISABLED;
>> +	spin_unlock_bh(&rq->lock);
>> +
>> +	return rc;
>> +}
>> +
>> +#else /* CONFIG_NET_RX_BUSY_POLL */
>> +static inline void virtnet_rq_init_lock(struct receive_queue *rq)
>> +{
>> +}
>> +
>> +static inline bool virtnet_rq_lock_napi_refill(struct receive_queue *rq)
>> +{
>> +	return true;
>> +}
>> +
>> +static inline bool virtnet_rq_unlock_napi_refill(struct receive_queue *rq)
>> +{
>> +	return false;
>> +}
>> +
>> +static inline bool virtnet_rq_lock_poll(struct receive_queue *rq)
>> +{
>> +	return false;
>> +}
>> +
>> +static inline bool virtnet_rq_unlock_poll(struct receive_queue *rq)
>> +{
>> +	return false;
>> +}
>> +
>> +static inline bool virtnet_rq_disable(struct receive_queue *rq)
>> +{
>> +	return true;
>> +}
>> +
>> +#endif /* CONFIG_NET_RX_BUSY_POLL */
>> +
>>  struct virtnet_info {
>>  	struct virtio_device *vdev;
>>  	struct virtqueue *cvq;
>> @@ -521,6 +657,8 @@ static void receive_buf(struct receive_queue *rq, void *buf, unsigned int len)
>>  		skb_shinfo(skb)->gso_segs = 0;
>>  	}
>>  
>> +	skb_mark_napi_id(skb, &rq->napi);
>> +
>>  	netif_receive_skb(skb);
>>  	return;
>>  
>> @@ -714,7 +852,12 @@ static void refill_work(struct work_struct *work)
>>  		struct receive_queue *rq = &vi->rq[i];
>>  
>>  		napi_disable(&rq->napi);
>> +		if (!virtnet_rq_lock_napi_refill(rq)) {
>> +			virtnet_napi_enable(rq);
>> +			continue;
>> +		}
>>  		still_empty = !try_fill_recv(rq, GFP_KERNEL);
>> +		virtnet_rq_unlock_napi_refill(rq);
>>  		virtnet_napi_enable(rq);
>>  
>>  		/* In theory, this can happen: if we don't get any buffers in
>> @@ -752,8 +895,13 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
>>  	unsigned int r, received = 0;
>>  
>>  again:
>> +	if (!virtnet_rq_lock_napi_refill(rq))
>> +		return budget;
>> +
>>  	received += virtnet_receive(rq, budget);
>>  
>> +	virtnet_rq_unlock_napi_refill(rq);
>> +
>>  	/* Out of packets? */
>>  	if (received < budget) {
>>  		r = virtqueue_enable_cb_prepare(rq->vq);
>> @@ -770,20 +918,50 @@ again:
>>  	return received;
>>  }
>>  
>> +#ifdef CONFIG_NET_RX_BUSY_POLL
>> +/* must be called with local_bh_disable()d */
>> +static int virtnet_low_latency_recv(struct napi_struct *napi)
> let's call it busy poll :)

Ok.
>> +{
>> +	struct receive_queue *rq =
>> +		container_of(napi, struct receive_queue, napi);
>> +	struct virtnet_info *vi = rq->vq->vdev->priv;
>> +	int received;
>> +
>> +	if (!(vi->status & VIRTIO_NET_S_LINK_UP))
>> +		return LL_FLUSH_FAILED;
>> +
>> +	if (!virtnet_rq_lock_poll(rq))
>> +		return LL_FLUSH_BUSY;
>> +
>> +	received = virtnet_receive(rq, 4);
> Hmm why 4 exactly?

I think the reason is we need a quota here to prevent the busy polling
method from starving other threads. 4 is just copied form the existed
implementation (ixgbe).
>> +
>> +	virtnet_rq_unlock_poll(rq);
>> +
>> +	return received;
>> +}
>> +#endif	/* CONFIG_NET_RX_BUSY_POLL */
>> +
>>  static void virtnet_napi_enable_all(struct virtnet_info *vi)
>>  {
>>  	int i;
>>  
>> -	for (i = 0; i < vi->max_queue_pairs; i++)
>> +	for (i = 0; i < vi->max_queue_pairs; i++) {
>> +		virtnet_rq_init_lock(&vi->rq[i]);
>>  		virtnet_napi_enable(&vi->rq[i]);
>> +	}
>>  }
>>  
>>  static void virtnet_napi_disable_all(struct virtnet_info *vi)
>>  {
>>  	int i;
>>  
>> -	for (i = 0; i < vi->max_queue_pairs; i++)
>> +	for (i = 0; i < vi->max_queue_pairs; i++) {
>>  		napi_disable(&vi->rq[i].napi);
>> +		while (!virtnet_rq_disable(&vi->rq[i])) {
>> +			pr_info("RQ %d locked\n", i);
>> +			usleep_range(1000, 20000);
> What's going on here, exactly?

It was used to wait for the completion of busy polling to finish.
>> +		}
>> +	}
>>  }
>>  
>>  static int virtnet_open(struct net_device *dev)
>> @@ -1372,6 +1550,9 @@ static const struct net_device_ops virtnet_netdev = {
>>  #ifdef CONFIG_NET_POLL_CONTROLLER
>>  	.ndo_poll_controller = virtnet_netpoll,
>>  #endif
>> +#ifdef CONFIG_NET_RX_BUSY_POLL
>> +	.ndo_busy_poll		= virtnet_low_latency_recv,
>> +#endif
>>  };
>>  
>>  static void virtnet_config_changed_work(struct work_struct *work)
>> @@ -1577,6 +1758,7 @@ static int virtnet_alloc_queues(struct virtnet_info *vi)
>>  		vi->rq[i].pages = NULL;
>>  		netif_napi_add(vi->dev, &vi->rq[i].napi, virtnet_poll,
>>  			       napi_weight);
>> +		napi_hash_add(&vi->rq[i].napi);
>>  
>>  		sg_init_table(vi->rq[i].sg, ARRAY_SIZE(vi->rq[i].sg));
>>  		ewma_init(&vi->rq[i].mrg_avg_pkt_len, 1, RECEIVE_AVG_WEIGHT);
>> @@ -1880,8 +2062,10 @@ static int virtnet_freeze(struct virtio_device *vdev)
>>  
>>  	if (netif_running(vi->dev)) {
>>  		virtnet_napi_disable_all(vi);
>> -		for (i = 0; i < vi->max_queue_pairs; i++)
>> +		for (i = 0; i < vi->max_queue_pairs; i++) {
>> +			napi_hash_del(&vi->rq[i].napi);
>>  			netif_napi_del(&vi->rq[i].napi);
>> +		}
>>  	}
>>  
>>  	remove_vq_common(vi);
>> -- 
>> 1.8.3.1

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