Subject: Re: [Patch net-next v3 2/3] net_sched: plug in qdisc ops change_tx_queue_len

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 26 Jan 2018 16:16:58 +0200

On Thu, Jan 25, 2018 at 06:26:23PM -0800, Cong Wang wrote:
> Introduce a new qdisc ops ->change_tx_queue_len() so that
> each qdisc could decide how to implement this if it wants.
> Previously we simply read dev->tx_queue_len, after pfifo_fast
> switches to skb array, we need this API to resize the skb array
> when we change dev->tx_queue_len.
> 
> To avoid handling race conditions with TX BH, we need to
> deactivate all TX queues before change the value and bring them
> back after we are done, this also makes implementation easier.
> 
> Cc: John Fastabend <[email-masked]>
> Signed-off-by: Cong Wang <[email-masked]>
> ---
>  include/net/sch_generic.h |  2 ++
>  net/core/dev.c            |  1 +
>  net/sched/sch_generic.c   | 33 +++++++++++++++++++++++++++++++++
>  3 files changed, 36 insertions(+)
> 
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index eac43e8ca96d..e2ab13687fb9 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -200,6 +200,7 @@ struct Qdisc_ops {
>  					  struct nlattr *arg,
>  					  struct netlink_ext_ack *extack);
>  	void			(*attach)(struct Qdisc *sch);
> +	int			(*change_tx_queue_len)(struct Qdisc *, unsigned int);
>  
>  	int			(*dump)(struct Qdisc *, struct sk_buff *);
>  	int			(*dump_stats)(struct Qdisc *, struct gnet_dump *);
> @@ -489,6 +490,7 @@ void qdisc_class_hash_remove(struct Qdisc_class_hash *,
>  void qdisc_class_hash_grow(struct Qdisc *, struct Qdisc_class_hash *);
>  void qdisc_class_hash_destroy(struct Qdisc_class_hash *);
>  
> +int dev_qdisc_change_tx_queue_len(struct net_device *dev);
>  void dev_init_scheduler(struct net_device *dev);
>  void dev_shutdown(struct net_device *dev);
>  void dev_activate(struct net_device *dev);
> diff --git a/net/core/dev.c b/net/core/dev.c
> index e0b0c2784070..c8443cfaa17a 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -7070,6 +7070,7 @@ int dev_change_tx_queue_len(struct net_device *dev, unsigned long new_len)
>  			dev->tx_queue_len = orig_len;
>  			return res;
>  		}
> +		return dev_qdisc_change_tx_queue_len(dev);
>  	}
>  
>  	return 0;
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 1816bde47256..08f9fa27e06e 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -1178,6 +1178,39 @@ void dev_deactivate(struct net_device *dev)
>  }
>  EXPORT_SYMBOL(dev_deactivate);
>  
> +static int qdisc_change_tx_queue_len(struct net_device *dev,
> +				     struct netdev_queue *dev_queue)
> +{
> +	struct Qdisc *qdisc = dev_queue->qdisc_sleeping;
> +	const struct Qdisc_ops *ops = qdisc->ops;
> +
> +	if (ops->change_tx_queue_len)
> +		return ops->change_tx_queue_len(qdisc, dev->tx_queue_len);
> +	return 0;
> +}
> +
> +int dev_qdisc_change_tx_queue_len(struct net_device *dev)
> +{
> +	bool up = dev->flags & IFF_UP;
> +	unsigned int i;
> +	int ret = 0;
> +
> +	if (up)
> +		dev_deactivate(dev);

This drops all packets in the queue. I don't think tweaking the queue
length did this previously - did it?
If not this change might surprise some people.

> +
> +	for (i = 0; i < dev->num_tx_queues; i++) {
> +		ret = qdisc_change_tx_queue_len(dev, &dev->_tx[i]);
> +
> +		/* TODO: revert changes on a partial failure */
> +		if (ret)
> +			break;
> +	}
> +
> +	if (up)
> +		dev_activate(dev);
> +	return ret;
> +}
> +
>  static void dev_init_scheduler_queue(struct net_device *dev,
>  				     struct netdev_queue *dev_queue,
>  				     void *_qdisc)
> -- 
> 2.13.0