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Date: Sun, 14 Mar 2021 08:04:03 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Jesper Dangaard Brouer <hawk@...nel.org>,
John Fastabend <john.fastabend@...il.com>,
virtualization@...ts.linux-foundation.org, bpf@...r.kernel.org
Subject: Re: [PATCH v8 net-next] virtio-net: support XDP when not more queues
On Wed, Mar 10, 2021 at 10:24:45AM +0800, Xuan Zhuo wrote:
> The number of queues implemented by many virtio backends is limited,
> especially some machines have a large number of CPUs. In this case, it
> is often impossible to allocate a separate queue for
> XDP_TX/XDP_REDIRECT, then xdp cannot be loaded to work, even xdp does
> not use the XDP_TX/XDP_REDIRECT.
>
> This patch allows XDP_TX/XDP_REDIRECT to run by reuse the existing SQ
> with __netif_tx_lock() hold when there are not enough queues.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Reviewed-by: Dust Li <dust.li@...ux.alibaba.com>
> Acked-by: Jason Wang <jasowang@...hat.com>
> ---
> v8: 1. explain why use macros not inline functions. (suggested by Michael S. Tsirkin)
> 2. empty line after variable definitions inside marcos. (suggested by Michael S. Tsirkin)
>
> v7: 1. use macros to implement get/put
> 2. remove 'flag'. (suggested by Jason Wang)
>
> v6: 1. use __netif_tx_acquire()/__netif_tx_release(). (suggested by Jason Wang)
> 2. add note for why not lock. (suggested by Jason Wang)
> 3. Use variable 'flag' to record with or without locked. It is not safe to
> use curr_queue_pairs in "virtnet_put_xdp_sq", because it may changed after
> "virtnet_get_xdp_sq".
>
> v5: change subject from 'support XDP_TX when not more queues'
>
> v4: make sparse happy
> suggested by Jakub Kicinski
>
> v3: add warning when no more queues
> suggested by Jesper Dangaard Brouer
>
> drivers/net/virtio_net.c | 62 ++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 49 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 82e520d..ae82e8e 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -195,6 +195,9 @@ struct virtnet_info {
> /* # of XDP queue pairs currently used by the driver */
> u16 xdp_queue_pairs;
>
> + /* xdp_queue_pairs may be 0, when xdp is already loaded. So add this. */
> + bool xdp_enabled;
> +
> /* I like... big packets and I cannot lie! */
> bool big_packets;
>
> @@ -481,12 +484,41 @@ static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
> return 0;
> }
>
> -static struct send_queue *virtnet_xdp_sq(struct virtnet_info *vi)
> -{
> - unsigned int qp;
> -
> - qp = vi->curr_queue_pairs - vi->xdp_queue_pairs + smp_processor_id();
> - return &vi->sq[qp];
> +/* when vi->curr_queue_pairs > nr_cpu_ids, the txq/sq is only used for xdp tx on
> + * the current cpu, so it does not need to be locked.
> + *
> + * Here we use marco instead of inline functions because we have to deal with
> + * three issues at the same time: 1. the choice of sq. 2. judge and execute the
> + * lock/unlock of txq 3. make sparse happy. It is difficult for two inline
> + * functions to perfectly solve these three problems at the same time.
This comment isn't really helpful :(
I did the following and sparse does not seem to complain.
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 2ca4bd2fec94..aee11164bab9 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -486,39 +486,36 @@ static int __virtnet_xdp_xmit_one(struct virtnet_info *vi,
/* when vi->curr_queue_pairs > nr_cpu_ids, the txq/sq is only used for xdp tx on
* the current cpu, so it does not need to be locked.
- *
- * Here we use marco instead of inline functions because we have to deal with
- * three issues at the same time: 1. the choice of sq. 2. judge and execute the
- * lock/unlock of txq 3. make sparse happy. It is difficult for two inline
- * functions to perfectly solve these three problems at the same time.
*/
-#define virtnet_xdp_get_sq(vi) ({ \
- struct netdev_queue *txq; \
- typeof(vi) v = (vi); \
- unsigned int qp; \
- \
- if (v->curr_queue_pairs > nr_cpu_ids) { \
- qp = v->curr_queue_pairs - v->xdp_queue_pairs; \
- qp += smp_processor_id(); \
- txq = netdev_get_tx_queue(v->dev, qp); \
- __netif_tx_acquire(txq); \
- } else { \
- qp = smp_processor_id() % v->curr_queue_pairs; \
- txq = netdev_get_tx_queue(v->dev, qp); \
- __netif_tx_lock(txq, raw_smp_processor_id()); \
- } \
- v->sq + qp; \
-})
+static inline struct send_queue *virtnet_xdp_get_sq(struct virtnet_info * vi)
+{
+ struct netdev_queue *txq;
+ typeof(vi) v = (vi);
+ unsigned int qp;
-#define virtnet_xdp_put_sq(vi, q) { \
- struct netdev_queue *txq; \
- typeof(vi) v = (vi); \
- \
- txq = netdev_get_tx_queue(v->dev, (q) - v->sq); \
- if (v->curr_queue_pairs > nr_cpu_ids) \
- __netif_tx_release(txq); \
- else \
- __netif_tx_unlock(txq); \
+ if (v->curr_queue_pairs > nr_cpu_ids) {
+ qp = v->curr_queue_pairs - v->xdp_queue_pairs;
+ qp += smp_processor_id();
+ txq = netdev_get_tx_queue(v->dev, qp);
+ __netif_tx_acquire(txq);
+ } else {
+ qp = smp_processor_id() % v->curr_queue_pairs;
+ txq = netdev_get_tx_queue(v->dev, qp);
+ __netif_tx_lock(txq, raw_smp_processor_id());
+ }
+ return v->sq + qp;
+}
+
+static inline void virtnet_xdp_put_sq(struct virtnet_info * vi, struct send_queue *q)
+{
+ struct netdev_queue *txq;
+ typeof(vi) v = (vi);
+
+ txq = netdev_get_tx_queue(v->dev, (q) - v->sq);
+ if (v->curr_queue_pairs > nr_cpu_ids)
+ __netif_tx_release(txq);
+ else
+ __netif_tx_unlock(txq);
}
static int virtnet_xdp_xmit(struct net_device *dev,
so what is the issue then?
> + */
> +#define virtnet_xdp_get_sq(vi) ({ \
> + struct netdev_queue *txq; \
> + typeof(vi) v = (vi); \
It's really always struct virtnet_info *vi isn't it?
Better use it as such so it's validated.
any local variables in a macro need to have very long names
otherwise it can shadow a local variable used in
a macro argument. E.g. __virtnet_xdp_get_sq_v.
> + unsigned int qp; \
I think it's just a tx queue index ... call it appropriately?
> + \
> + if (v->curr_queue_pairs > nr_cpu_ids) { \
> + qp = v->curr_queue_pairs - v->xdp_queue_pairs; \
> + qp += smp_processor_id(); \
> + txq = netdev_get_tx_queue(v->dev, qp); \
> + __netif_tx_acquire(txq); \
> + } else { \
> + qp = smp_processor_id() % v->curr_queue_pairs; \
> + txq = netdev_get_tx_queue(v->dev, qp); \
> + __netif_tx_lock(txq, raw_smp_processor_id()); \
> + } \
> + v->sq + qp; \
> +})
> +
> +#define virtnet_xdp_put_sq(vi, q) { \
> + struct netdev_queue *txq; \
> + typeof(vi) v = (vi); \
> + \
> + txq = netdev_get_tx_queue(v->dev, (q) - v->sq); \
> + if (v->curr_queue_pairs > nr_cpu_ids) \
> + __netif_tx_release(txq); \
> + else \
> + __netif_tx_unlock(txq); \
can curr_queue_pairs change after the call to virtnet_xdp_get_sq?
If it does the lock/unlock won't be balanced ...
pls add a comment explaining why that can't happen ...
or maybe better yet, just return the tx queue number from get and
pass it to put?
> }
>
> static int virtnet_xdp_xmit(struct net_device *dev,
> @@ -512,7 +544,7 @@ static int virtnet_xdp_xmit(struct net_device *dev,
> if (!xdp_prog)
> return -ENXIO;
>
> - sq = virtnet_xdp_sq(vi);
> + sq = virtnet_xdp_get_sq(vi);
>
> if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK)) {
> ret = -EINVAL;
> @@ -560,12 +592,13 @@ static int virtnet_xdp_xmit(struct net_device *dev,
> sq->stats.kicks += kicks;
> u64_stats_update_end(&sq->stats.syncp);
>
> + virtnet_xdp_put_sq(vi, sq);
> return ret;
> }
>
> static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
> {
> - return vi->xdp_queue_pairs ? VIRTIO_XDP_HEADROOM : 0;
> + return vi->xdp_enabled ? VIRTIO_XDP_HEADROOM : 0;
> }
>
> /* We copy the packet for XDP in the following cases:
> @@ -1458,12 +1491,13 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
> xdp_do_flush();
>
> if (xdp_xmit & VIRTIO_XDP_TX) {
> - sq = virtnet_xdp_sq(vi);
> + sq = virtnet_xdp_get_sq(vi);
> if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq)) {
> u64_stats_update_begin(&sq->stats.syncp);
> sq->stats.kicks++;
> u64_stats_update_end(&sq->stats.syncp);
> }
> + virtnet_xdp_put_sq(vi, sq);
> }
>
> return received;
> @@ -2418,10 +2452,9 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>
> /* XDP requires extra queues for XDP_TX */
> if (curr_qp + xdp_qp > vi->max_queue_pairs) {
> - NL_SET_ERR_MSG_MOD(extack, "Too few free TX rings available");
> - netdev_warn(dev, "request %i queues but max is %i\n",
> + netdev_warn(dev, "XDP request %i queues but max is %i. XDP_TX and XDP_REDIRECT will operate in a slower locked tx mode.\n",
> curr_qp + xdp_qp, vi->max_queue_pairs);
> - return -ENOMEM;
> + xdp_qp = 0;
> }
>
> old_prog = rtnl_dereference(vi->rq[0].xdp_prog);
> @@ -2455,11 +2488,14 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> vi->xdp_queue_pairs = xdp_qp;
>
> if (prog) {
> + vi->xdp_enabled = true;
> for (i = 0; i < vi->max_queue_pairs; i++) {
> rcu_assign_pointer(vi->rq[i].xdp_prog, prog);
> if (i == 0 && !old_prog)
> virtnet_clear_guest_offloads(vi);
> }
> + } else {
> + vi->xdp_enabled = false;
> }
>
> for (i = 0; i < vi->max_queue_pairs; i++) {
> @@ -2527,7 +2563,7 @@ static int virtnet_set_features(struct net_device *dev,
> int err;
>
> if ((dev->features ^ features) & NETIF_F_LRO) {
> - if (vi->xdp_queue_pairs)
> + if (vi->xdp_enabled)
> return -EBUSY;
>
> if (features & NETIF_F_LRO)
> --
> 1.8.3.1
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