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

Michael S. Tsirkin
2017-10-18T14:12:17Z

On Mon, Oct 16, 2017 at 12:19:39PM +0200, Jesper Dangaard Brouer wrote:
> @@ -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)) {

I suspect this is racy: if ring becomes non empty here and
you wake the task, next line will put it to sleep.
I think you want to reverse the order:

__set_current_state(TASK_INTERRUPTIBLE);

then check __ptr_ring_empty.

I note using the __ version means you can not resize the ring.
Hope you do not need to.

> + __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;