[net-next V8 PATCH 3/5] bpf: cpumap xdp_buff to skb conversion and allocation

Jesper Dangaard Brouer
2017-10-16T10:19:40Z

This patch makes cpumap functional, by adding SKB allocation and
invoking the network stack on the dequeuing CPU.

For constructing the SKB on the remote CPU, the xdp_buff in converted
into a struct xdp_pkt, and it mapped into the top headroom of the
packet, to avoid allocating separate mem. For now, struct xdp_pkt is
just a cpumap internal data structure, with info carried between
enqueue to dequeue.

If a driver doesn't have enough headroom it is simply dropped, with
return code -EOVERFLOW. This will be picked up the xdp tracepoint
infrastructure, to allow users to catch this.

V2: take into account xdp->data_meta

V4:
- Drop busypoll tricks, keeping it more simple.
- Skip RPS and Generic-XDP-recursive-reinjection, suggested by Alexei

V5: correct RCU read protection around __netif_receive_skb_core.

V6: Setting TASK_RUNNING vs TASK_INTERRUPTIBLE based on talk with Rik van Riel

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
include/linux/netdevice.h | 1
kernel/bpf/cpumap.c | 152 ++++++++++++++++++++++++++++++++++++++-------
net/core/dev.c | 27 ++++++++
3 files changed, 158 insertions(+), 22 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 31bb3010c69b..bf014afcb914 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3260,6 +3260,7 @@ int do_xdp_generic(struct bpf_prog *xdp_prog, struct sk_buff *skb);
int netif_rx(struct sk_buff *skb);
int netif_rx_ni(struct sk_buff *skb);
int netif_receive_skb(struct sk_buff *skb);
+int netif_receive_skb_core(struct sk_buff *skb);
gro_result_t napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb);
void napi_gro_flush(struct napi_struct *napi, bool flush_old);
struct sk_buff *napi_get_frags(struct napi_struct *napi);
diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
index 768da6a2c265..ee7adf4352dd 100644
--- a/kernel/bpf/cpumap.c
+++ b/kernel/bpf/cpumap.c
@@ -25,6 +25,9 @@
#include <linux/kthread.h>
#include <linux/capability.h>

+#include <linux/netdevice.h> /* netif_receive_skb_core */
+#include <linux/etherdevice.h> /* eth_type_trans */
+
/* General idea: XDP packets getting XDP redirected to another CPU,
* will maximum be stored/queued for one driver ->poll() call. It is
* guaranteed that setting flush bit and flush operation happen on
@@ -179,6 +182,92 @@ static void cpu_map_kthread_stop(struct work_struct *work)
kthread_stop(rcpu->kthread);
}

+/* For now, xdp_pkt is a cpumap internal data structure, with info
+ * carried between enqueue to dequeue. It is mapped into the top
+ * headroom of the packet, to avoid allocating separate mem.
+ */
+struct xdp_pkt {
+ void *data;
+ u16 len;
+ u16 headroom;
+ u16 metasize;
+ struct net_device *dev_rx;
+};
+
+/* Convert xdp_buff to xdp_pkt */
+static struct xdp_pkt *convert_to_xdp_pkt(struct xdp_buff *xdp)
+{
+ struct xdp_pkt *xdp_pkt;
+ int metasize;
+ int headroom;
+
+ /* Assure headroom is available for storing info */
+ headroom = xdp->data - xdp->data_hard_start;
+ metasize = xdp->data - xdp->data_meta;
+ metasize = metasize > 0 ? metasize : 0;
+ if ((headroom - metasize) < sizeof(*xdp_pkt))
+ return NULL;
+
+ /* Store info in top of packet */
+ xdp_pkt = xdp->data_hard_start;
+
+ xdp_pkt->data = xdp->data;
+ xdp_pkt->len = xdp->data_end - xdp->data;
+ xdp_pkt->headroom = headroom - sizeof(*xdp_pkt);
+ xdp_pkt->metasize = metasize;
+
+ return xdp_pkt;
+}
+
+struct sk_buff *cpu_map_build_skb(struct bpf_cpu_map_entry *rcpu,
+ struct xdp_pkt *xdp_pkt)
+{
+ unsigned int frame_size;
+ void *pkt_data_start;
+ struct sk_buff *skb;
+
+ /* build_skb need to place skb_shared_info after SKB end, and
+ * also want to know the memory "truesize". Thus, need to
+ * know the memory frame size backing xdp_buff.
+ *
+ * XDP was designed to have PAGE_SIZE frames, but this
+ * assumption is not longer true with ixgbe and i40e. It
+ * would be preferred to set frame_size to 2048 or 4096
+ * depending on the driver.
+ * frame_size = 2048;
+ * frame_len = frame_size - sizeof(*xdp_pkt);
+ *
+ * Instead, with info avail, skb_shared_info in placed after
+ * packet len. This, unfortunately fakes the truesize.
+ * Another disadvantage of this approach, the skb_shared_info
+ * is not at a fixed memory location, with mixed length
+ * packets, which is bad for cache-line hotness.
+ */
+ frame_size = SKB_DATA_ALIGN(xdp_pkt->len) + xdp_pkt->headroom +
+ SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
+
+ pkt_data_start = xdp_pkt->data - xdp_pkt->headroom;
+ skb = build_skb(pkt_data_start, frame_size);
+ if (!skb)
+ return NULL;
+
+ skb_reserve(skb, xdp_pkt->headroom);
+ __skb_put(skb, xdp_pkt->len);
+ if (xdp_pkt->metasize)
+ skb_metadata_set(skb, xdp_pkt->metasize);
+
+ /* Essential SKB info: protocol and skb->dev */
+ skb->protocol = eth_type_trans(skb, xdp_pkt->dev_rx);
+
+ /* Optional SKB info, currently missing:
+ * - HW checksum info (skb->ip_summed)
+ * - HW RX hash (skb_set_hash)
+ * - RX ring dev queue index (skb_record_rx_queue)
+ */
+
+ return skb;
+}
+
static int cpu_map_kthread_run(void *data)
{
struct bpf_cpu_map_entry *rcpu = data;
@@ -191,15 +280,45 @@ static int cpu_map_kthread_run(void *data)
* kthread_stop signal until queue is empty.
*/
while (!kthread_should_stop() || !__ptr_ring_empty(rcpu->queue)) {
+ unsigned int processed = 0, drops = 0;
struct xdp_pkt *xdp_pkt;

- schedule();
- /* Do work */
- while ((xdp_pkt = ptr_ring_consume(rcpu->queue))) {
- /* For now just "refcnt-free" */
- page_frag_free(xdp_pkt);
+ /* Release CPU reschedule checks */
+ if (__ptr_ring_empty(rcpu->queue)) {
+ __set_current_state(TASK_INTERRUPTIBLE);
+ schedule();
+ } else {
+ cond_resched();
+ }
+ __set_current_state(TASK_RUNNING);
+
+ /* Process packets in rcpu->queue */
+ local_bh_disable();
+ /*
+ * The bpf_cpu_map_entry is single consumer, with this
+ * kthread CPU pinned. Lockless access to ptr_ring
+ * consume side valid as no-resize allowed of queue.
+ */
+ while ((xdp_pkt = __ptr_ring_consume(rcpu->queue))) {
+ struct sk_buff *skb;
+ int ret;
+
+ skb = cpu_map_build_skb(rcpu, xdp_pkt);
+ if (!skb) {
+ page_frag_free(xdp_pkt);
+ continue;
+ }
+
+ /* Inject into network stack */
+ ret = netif_receive_skb_core(skb);
+ if (ret == NET_RX_DROP)
+ drops++;
+
+ /* Limit BH-disable period */
+ if (++processed == 8)
+ break;
}
- __set_current_state(TASK_INTERRUPTIBLE);
+ local_bh_enable(); /* resched point, may call do_softirq() */
}
__set_current_state(TASK_RUNNING);

@@ -490,13 +609,6 @@ static int bq_flush_to_queue(struct bpf_cpu_map_entry *rcpu,
return 0;
}

-/* Notice: Will change in later patch */
-struct xdp_pkt {
- void *data;
- u16 len;
- u16 headroom;
-};
-
/* Runs under RCU-read-side, plus in softirq under NAPI protection.
* Thus, safe percpu variable access.
*/
@@ -524,17 +636,13 @@ int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_buff *xdp,
struct net_device *dev_rx)
{
struct xdp_pkt *xdp_pkt;
- int headroom;

- /* For now this is just used as a void pointer to data_hard_start.
- * Followup patch will generalize this.
- */
- xdp_pkt = xdp->data_hard_start;
+ xdp_pkt = convert_to_xdp_pkt(xdp);
+ if (!xdp_pkt)
+ return -EOVERFLOW;

- /* Fake writing into xdp_pkt->data to measure overhead */
- headroom = xdp->data - xdp->data_hard_start;
- if (headroom < sizeof(*xdp_pkt))
- xdp_pkt->data = xdp->data;
+ /* Info needed when constructing SKB on remote CPU */
+ xdp_pkt->dev_rx = dev_rx;

bq_enqueue(rcpu, xdp_pkt);
return 0;
diff --git a/net/core/dev.c b/net/core/dev.c
index fcddccb6be41..36bb68f3a2c7 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4491,6 +4491,33 @@ static int __netif_receive_skb_core(struct sk_buff *skb, bool pfmemalloc)
return ret;
}

+/**
+ * netif_receive_skb_core - special purpose version of netif_receive_skb
+ * @skb: buffer to process
+ *
+ * More direct receive version of netif_receive_skb(). It should
+ * only be used by callers that have a need to skip RPS and Generic XDP.
+ * Caller must also take care of handling if (page_is_)pfmemalloc.
+ *
+ * This function may only be called from softirq context and interrupts
+ * should be enabled.
+ *
+ * Return values (usually ignored):
+ * NET_RX_SUCCESS: no congestion
+ * NET_RX_DROP: packet was dropped
+ */
+int netif_receive_skb_core(struct sk_buff *skb)
+{
+ int ret;
+
+ rcu_read_lock();
+ ret = __netif_receive_skb_core(skb, false);
+ rcu_read_unlock();
+
+ return ret;
+}
+EXPORT_SYMBOL(netif_receive_skb_core);
+
static int __netif_receive_skb(struct sk_buff *skb)
{
int ret;