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

Jesper Dangaard Brouer
2017-10-19T10:11:02Z

On Wed, 18 Oct 2017 17:12:09 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> 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'll look into this.

The window will be minimal, as __cpu_map_flush() after the last packets
enqueue will call wake_up_process(rcpu->kthread). But I guess there is
still small race possible. Worst case, a packet could be stuck in the
queue until a new packet arrive. Thanks for spotting this.

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

Resize is not supported. If user change the queue size, a new ptr_ring
and kthread is created, and logic assured the old ptr_ring and kthread
flush packets appropriately (this is tested with the --stress-mode).

> > + __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);
> >

--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer