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Message-ID: <20110602144346.GA7995@redhat.com>
Date: Thu, 2 Jun 2011 17:43:46 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Krishna Kumar2 <krkumar2@...ibm.com>
Cc: Christian Borntraeger <borntraeger@...ibm.com>,
Carsten Otte <cotte@...ibm.com>, habanero@...ux.vnet.ibm.com,
Heiko Carstens <heiko.carstens@...ibm.com>,
kvm@...r.kernel.org, lguest@...ts.ozlabs.org,
linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
linux390@...ibm.com, netdev@...r.kernel.org,
Rusty Russell <rusty@...tcorp.com.au>,
Martin Schwidefsky <schwidefsky@...ibm.com>, steved@...ibm.com,
Tom Lendacky <tahm@...ux.vnet.ibm.com>,
virtualization@...ts.linux-foundation.org,
Shirley Ma <xma@...ibm.com>
Subject: Re: [PATCH RFC 3/3] virtio_net: limit xmit polling
On Thu, Jun 02, 2011 at 07:47:48PM +0530, Krishna Kumar2 wrote:
> > OK, I have something very similar, but I still dislike the screw the
> > latency part: this path is exactly what the IBM guys seem to hit. So I
> > created two functions: one tries to free a constant number and another
> > one up to capacity. I'll post that now.
>
> Please review this patch to see if it looks reasonable:
Hmm, since you decided to work on top of my patch,
I'd appreciate split-up fixes.
> 1. Picked comments/code from MST's code and Rusty's review.
> 2. virtqueue_min_capacity() needs to be called only if it returned
> empty the last time it was called.
> 3. Fix return value bug in free_old_xmit_skbs (hangs guest).
> 4. Stop queue only if capacity is not enough for next xmit.
That's what we always did ...
> 5. Fix/clean some likely/unlikely checks (hopefully).
>
> I have done some minimal netperf tests with this.
>
> With this patch, add_buf returning capacity seems to be useful - it
> allows less virtio API calls.
Why bother? It's cheap ...
>
> Signed-off-by: Krishna Kumar <krkumar2@...ibm.com>
> ---
> drivers/net/virtio_net.c | 105 ++++++++++++++++++++++---------------
> 1 file changed, 64 insertions(+), 41 deletions(-)
>
> diff -ruNp org/drivers/net/virtio_net.c new/drivers/net/virtio_net.c
> --- org/drivers/net/virtio_net.c 2011-06-02 15:49:25.000000000 +0530
> +++ new/drivers/net/virtio_net.c 2011-06-02 19:13:02.000000000 +0530
> @@ -509,27 +509,43 @@ again:
> return received;
> }
>
> -/* Check capacity and try to free enough pending old buffers to enable queueing
> - * new ones. If min_skbs > 0, try to free at least the specified number of skbs
> - * even if the ring already has sufficient capacity. Return true if we can
> - * guarantee that a following virtqueue_add_buf will succeed. */
> -static bool free_old_xmit_skbs(struct virtnet_info *vi, int min_skbs)
> +/* Return true if freed a skb, else false */
> +static inline bool free_one_old_xmit_skb(struct virtnet_info *vi)
> {
> struct sk_buff *skb;
> unsigned int len;
> - bool r;
>
> - while ((r = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2) ||
> - min_skbs-- > 0) {
> - skb = virtqueue_get_buf(vi->svq, &len);
> - if (unlikely(!skb))
> + skb = virtqueue_get_buf(vi->svq, &len);
> + if (unlikely(!skb))
> + return 0;
> +
> + 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 1;
> +}
> +
> +static bool free_old_xmit_skbs(struct virtnet_info *vi, int to_free)
> +{
> + bool empty = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2;
> +
> + do {
> + if (!free_one_old_xmit_skb(vi)) {
> + /* No more skbs to free up */
> break;
> - 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 r;
> + }
> +
> + if (empty) {
> + /* Check again if there is enough space */
> + empty = virtqueue_min_capacity(vi->svq) <
> + MAX_SKB_FRAGS + 2;
> + } else {
> + --to_free;
> + }
> + } while (to_free > 0);
> +
> + return !empty;
> }
Why bother doing the capacity check in this function?
> static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
> @@ -582,46 +598,53 @@ static int xmit_skb(struct virtnet_info
> static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> {
> struct virtnet_info *vi = netdev_priv(dev);
> - int ret, n;
> + int capacity;
>
> - /* Free up space in the ring in case this is the first time we get
> - * woken up after ring full condition. Note: this might try to free
> - * more than strictly necessary if the skb has a small
> - * number of fragments, but keep it simple. */
> - free_old_xmit_skbs(vi, 0);
> + /* Try to free 2 buffers for every 1 xmit, to stay ahead. */
> + free_old_xmit_skbs(vi, 2);
>
> /* Try to transmit */
> - ret = xmit_skb(vi, skb);
> + capacity = xmit_skb(vi, skb);
>
> - /* Failure to queue is unlikely. It's not a bug though: it might happen
> - * if we get an interrupt while the queue is still mostly full.
> - * We could stop the queue and re-enable callbacks (and possibly return
> - * TX_BUSY), but as this should be rare, we don't bother. */
> - if (unlikely(ret < 0)) {
> + if (unlikely(capacity < 0)) {
> + /*
> + * Failure to queue should be impossible. The only way to
> + * reach here is if we got a cb before 3/4th of space was
> + * available. We could stop the queue and re-enable
> + * callbacks (and possibly return TX_BUSY), but we don't
> + * bother since this is impossible.
It's far from impossible. The 3/4 thing is only a hint, and old devices
don't support it anyway.
> + */
> if (net_ratelimit())
> - dev_info(&dev->dev, "TX queue failure: %d\n", ret);
> + dev_info(&dev->dev, "TX queue failure: %d\n", capacity);
> dev->stats.tx_dropped++;
> kfree_skb(skb);
> return NETDEV_TX_OK;
> }
> +
> virtqueue_kick(vi->svq);
>
> /* Don't wait up for transmitted skbs to be freed. */
> skb_orphan(skb);
> nf_reset(skb);
>
> - /* Apparently nice girls don't return TX_BUSY; check capacity and stop
> - * the queue before it gets out of hand.
> - * Naturally, this wastes entries. */
> - /* 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. */
> - if (!likely(free_old_xmit_skbs(vi, 2))) {
> - netif_stop_queue(dev);
> - if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> - /* More just got used, free them and recheck. */
> - if (!likely(free_old_xmit_skbs(vi, 0))) {
> - netif_start_queue(dev);
> - virtqueue_disable_cb(vi->svq);
> + /*
> + * Apparently nice girls don't return TX_BUSY; check capacity and
> + * stop the queue before it gets out of hand. Naturally, this wastes
> + * entries.
> + */
> + if (capacity < 2+MAX_SKB_FRAGS) {
> + /*
> + * We don't have enough space for the next packet. Try
> + * freeing more.
> + */
> + if (likely(!free_old_xmit_skbs(vi, UINT_MAX))) {
> + netif_stop_queue(dev);
> + if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> + /* More just got used, free them and recheck. */
> + if (likely(free_old_xmit_skbs(vi, UINT_MAX))) {
Is this where the bug was?
> + netif_start_queue(dev);
> + virtqueue_disable_cb(vi->svq);
> + }
> }
> }
> }
--
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