netdev - Re: [PATCH net-next v10 2/3] xen networking: add basic XDP support for xen-netfront

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Date:   Mon, 22 Jun 2020 17:27:01 +0300
From:   Denis Kirjanov <kda@...ux-powerpc.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, jgross@...e.com, wei.liu@...nel.org,
        paul@....org, ilias.apalodimas@...aro.org
Subject: Re: [PATCH net-next v10 2/3] xen networking: add basic XDP support
 for xen-netfront

On 6/22/20, Jesper Dangaard Brouer <brouer@...hat.com> wrote:
> On Mon, 22 Jun 2020 15:45:46 +0300
> Denis Kirjanov <kda@...ux-powerpc.org> wrote:
>
>> On 6/22/20, Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>> >
>> > On Mon, 22 Jun 2020 12:21:11 +0300 Denis Kirjanov
>> > <kda@...ux-powerpc.org>
>> > wrote:
>> >
>> >> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
>> >> index 482c6c8..1b9f49e 100644
>> >> --- a/drivers/net/xen-netfront.c
>> >> +++ b/drivers/net/xen-netfront.c
>> > [...]
>> >> @@ -560,6 +572,65 @@ static u16 xennet_select_queue(struct net_device
>> >> *dev, struct sk_buff *skb,
>> >>  	return queue_idx;
>> >>  }
>> >>
>> >> +static int xennet_xdp_xmit_one(struct net_device *dev, struct
>> >> xdp_frame
>> >> *xdpf)
>> >> +{
>> >> +	struct netfront_info *np = netdev_priv(dev);
>> >> +	struct netfront_stats *tx_stats = this_cpu_ptr(np->tx_stats);
>> >> +	unsigned int num_queues = dev->real_num_tx_queues;
>> >> +	struct netfront_queue *queue = NULL;
>> >> +	struct xen_netif_tx_request *tx;
>> >> +	unsigned long flags;
>> >> +	int notify;
>> >> +
>> >> +	queue = &np->queues[smp_processor_id() % num_queues];
>> >> +
>> >> +	spin_lock_irqsave(&queue->tx_lock, flags);
>> >
>> > Why are you taking a lock per packet (xdp_frame)?
>> Hi Jesper,
>>
>> We have to protect shared ring indices.
>
> Sure, I understand we need to protect the rings.
>
> What I'm asking is why are doing this per-packet, and not once for the
> entire bulk of packets?

Now I see. I believe we can. Do you think it will give performance
from the cache perspective?
>
> (notice how xennet_xdp_xmit gets a bulk of packets)
>
>> >
>> >> +
>> >> +	tx = xennet_make_first_txreq(queue, NULL,
>> >> +				     virt_to_page(xdpf->data),
>> >> +				     offset_in_page(xdpf->data),
>> >> +				     xdpf->len);
>> >> +
>> >> +	RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&queue->tx, notify);
>> >> +	if (notify)
>> >> +		notify_remote_via_irq(queue->tx_irq);
>> >> +
>> >> +	u64_stats_update_begin(&tx_stats->syncp);
>> >> +	tx_stats->bytes += xdpf->len;
>> >> +	tx_stats->packets++;
>> >> +	u64_stats_update_end(&tx_stats->syncp);
>> >> +
>> >> +	xennet_tx_buf_gc(queue);
>> >> +
>> >> +	spin_unlock_irqrestore(&queue->tx_lock, flags);
>> >
>> > Is the irqsave/irqrestore variant really needed here?
>>
>> netpoll also invokes the tx completion handler.
>
> I forgot about netpoll.
>
> The netpoll code cannot call this code path xennet_xdp_xmit /
> xennet_xdp_xmit_one, right?
>
> Are the per-CPU ring queue's shared with normal network stack, that can
> be called from netpoll code path?

I meant that both xennet_start_xmit and xennet_poll_controller call
xennet_tx_buf_gc

>
>   queue = &np->queues[smp_processor_id() % num_queues];
>
>
>> >
>> >> +	return 0;
>> >> +}
>> >> +
>> >> +static int xennet_xdp_xmit(struct net_device *dev, int n,
>> >> +			   struct xdp_frame **frames, u32 flags)
>> >> +{
>> >> +	int drops = 0;
>> >> +	int i, err;
>> >> +
>> >> +	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK))
>> >> +		return -EINVAL;
>> >> +
>> >> +	for (i = 0; i < n; i++) {
>> >> +		struct xdp_frame *xdpf = frames[i];
>> >> +
>> >> +		if (!xdpf)
>> >> +			continue;
>> >> +		err = xennet_xdp_xmit_one(dev, xdpf);
>> >> +		if (err) {
>> >> +			xdp_return_frame_rx_napi(xdpf);
>> >> +			drops++;
>> >> +		}
>> >> +	}
>> >> +
>> >> +	return n - drops;
>> >> +}
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
>
>