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Message-ID: <20110602192357.GC13444@redhat.com>
Date: Thu, 2 Jun 2011 22:23:57 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Sridhar Samudrala <sri@...ibm.com>
Cc: linux-kernel@...r.kernel.org,
Rusty Russell <rusty@...tcorp.com.au>,
Carsten Otte <cotte@...ibm.com>,
Christian Borntraeger <borntraeger@...ibm.com>,
linux390@...ibm.com, Martin Schwidefsky <schwidefsky@...ibm.com>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Shirley Ma <xma@...ibm.com>, lguest@...ts.ozlabs.org,
virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
linux-s390@...r.kernel.org, kvm@...r.kernel.org,
Krishna Kumar <krkumar2@...ibm.com>,
Tom Lendacky <tahm@...ux.vnet.ibm.com>, steved@...ibm.com,
habanero@...ux.vnet.ibm.com
Subject: Re: [PATCHv2 RFC 3/4] virtio_net: limit xmit polling
On Thu, Jun 02, 2011 at 11:09:53AM -0700, Sridhar Samudrala wrote:
> On Thu, 2011-06-02 at 18:43 +0300, Michael S. Tsirkin wrote:
> > Current code might introduce a lot of latency variation
> > if there are many pending bufs at the time we
> > attempt to transmit a new one. This is bad for
> > real-time applications and can't be good for TCP either.
> >
> > Free up just enough to both clean up all buffers
> > eventually and to be able to xmit the next packet.
> >
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> > drivers/net/virtio_net.c | 106 +++++++++++++++++++++++++++++-----------------
> > 1 files changed, 67 insertions(+), 39 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index a0ee78d..b25db1c 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -509,17 +509,33 @@ again:
> > return received;
> > }
> >
> > -static void free_old_xmit_skbs(struct virtnet_info *vi)
> > +static bool free_old_xmit_skb(struct virtnet_info *vi)
> > {
> > struct sk_buff *skb;
> > unsigned int len;
> >
> > - while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
> > - pr_debug("Sent skb %p\n", skb);
> > - vi->dev->stats.tx_bytes += skb->len;
> > - vi->dev->stats.tx_packets++;
> > - dev_kfree_skb_any(skb);
> > - }
> > + skb = virtqueue_get_buf(vi->svq, &len);
> > + if (unlikely(!skb))
> > + return false;
> > + pr_debug("Sent skb %p\n", skb);
> > + vi->dev->stats.tx_bytes += skb->len;
> > + vi->dev->stats.tx_packets++;
> > + dev_kfree_skb_any(skb);
> > + return true;
> > +}
> > +
> > +/* Check capacity and try to free enough pending old buffers to enable queueing
> > + * new ones. Return true if we can guarantee that a following
> > + * virtqueue_add_buf will succeed. */
> > +static bool free_xmit_capacity(struct virtnet_info *vi)
> > +{
> > + struct sk_buff *skb;
> > + unsigned int len;
> > +
> > + while (virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2)
> > + if (unlikely(!free_old_xmit_skb))
> > + return false;
> If we are using INDIRECT descriptors, 1 descriptor entry is good enough
> to guarantee that an skb can be queued unless we run out of memory.
> Is it worth checking if 'indirect' is set on the svq and then only free
> 1 descriptor? Otherwise, we will be dropping the packet if there are
> less than 18 free descriptors although we ony need 1.
This is not introduced with this patch: the
issue is that we need to consider worst case
as indirect memory allocation can fail.
I don't think it matters much: 18 out of 256
descriptors is not too expensive.
> > + return true;
> > }
> >
> > static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
> > @@ -572,30 +588,34 @@ static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
> > static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > {
> > struct virtnet_info *vi = netdev_priv(dev);
> > - int capacity;
> > -
> > - /* Free up any pending old buffers before queueing new ones. */
> > - free_old_xmit_skbs(vi);
> > -
> > - /* Try to transmit */
> > - capacity = xmit_skb(vi, skb);
> > -
> > - /* This can happen with OOM and indirect buffers. */
> > - if (unlikely(capacity < 0)) {
> > - if (net_ratelimit()) {
> > - if (likely(capacity == -ENOMEM)) {
> > - dev_warn(&dev->dev,
> > - "TX queue failure: out of memory\n");
> > - } else {
> > - dev->stats.tx_fifo_errors++;
> > + int ret, i;
> > +
> > + /* We normally do have space in the ring, so try to queue the skb as
> > + * fast as possible. */
> > + ret = xmit_skb(vi, skb);
> > + if (unlikely(ret < 0)) {
> > + /* This triggers on the first xmit after ring full condition.
> > + * We need to free up some skbs first. */
> > + if (likely(free_xmit_capacity(vi))) {
> > + ret = xmit_skb(vi, skb);
> > + /* This should never fail. Check, just in case. */
> > + if (unlikely(ret < 0)) {
> > dev_warn(&dev->dev,
> > "Unexpected TX queue failure: %d\n",
> > - capacity);
> > + ret);
> > + dev->stats.tx_fifo_errors++;
> > + dev->stats.tx_dropped++;
> > + kfree_skb(skb);
> > + return NETDEV_TX_OK;
> > }
> > + } else {
> > + /* Ring full: it might happen if we get a callback while
> > + * the queue is still mostly full. This should be
> > + * extremely rare. */
> > + dev->stats.tx_dropped++;
> > + kfree_skb(skb);
> > + goto stop;
> > }
> > - dev->stats.tx_dropped++;
> > - kfree_skb(skb);
> > - return NETDEV_TX_OK;
> > }
> > virtqueue_kick(vi->svq);
> >
> > @@ -603,18 +623,26 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > skb_orphan(skb);
> > nf_reset(skb);
> >
> > - /* Apparently nice girls don't return TX_BUSY; stop the queue
> > - * before it gets out of hand. Naturally, this wastes entries. */
> > - if (capacity < 2+MAX_SKB_FRAGS) {
> > - netif_stop_queue(dev);
> > - if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> > - /* More just got used, free them then recheck. */
> > - free_old_xmit_skbs(vi);
> > - capacity = virtqueue_min_capacity(vi->svq);
> > - if (capacity >= 2+MAX_SKB_FRAGS) {
> > - netif_start_queue(dev);
> > - virtqueue_disable_cb(vi->svq);
> > - }
> > + /* We transmit one skb, so try to free at least two pending skbs.
> > + * This is so that we don't hog the skb memory unnecessarily. *
> > + * Doing this after kick means there's a chance we'll free
> > + * the skb we have just sent, which is hot in cache. */
> > + for (i = 0; i < 2; i++)
> > + free_old_xmit_skb(v);
> > +
> > + if (likely(free_xmit_capacity(vi)))
> > + return NETDEV_TX_OK;
> > +
> > +stop:
> > + /* Apparently nice girls don't return TX_BUSY; check capacity and stop
> > + * the queue before it gets out of hand.
> > + * Naturally, this wastes entries. */
> > + netif_stop_queue(dev);
> > + if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> > + /* More just got used, free them and recheck. */
> > + if (free_xmit_capacity(vi)) {
> > + netif_start_queue(dev);
> > + virtqueue_disable_cb(vi->svq);
> > }
> > }
> >
--
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