Date:	Wed, 17 Nov 2010 07:56:30 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	John Fastabend <john.r.fastabend@...el.com>
Cc:	netdev@...r.kernel.org, nhorman@...driver.com, davem@...emloft.net
Subject: Re: [RFC PATCH v1 1/2] net: implement mechanism for HW based QOS

Le mardi 16 novembre 2010 à 21:15 -0800, John Fastabend a écrit :
> This patch provides a mechanism for lower layer devices to
> steer traffic using skb->priority to tx queues. This allows
> for hardware based QOS schemes to use the default qdisc without
> incurring the penalties related to global state and the qdisc
> lock. While reliably receiving skbs on the correct tx ring
> to avoid head of line blocking resulting from shuffling in
> the LLD. Finally, all the goodness from txq caching and xps/rps
> can still be leveraged.
> 
> Many drivers and hardware exist with the ability to implement
> QOS schemes in the hardware but currently these drivers tend
> to rely on firmware to reroute specific traffic, a driver
> specific select_queue or the queue_mapping action in the
> qdisc.
> 
> None of these solutions are ideal or generic so we end up
> with driver specific solutions that one-off traffic types
> for example FCoE traffic is steered in ixgbe with the
> queue_select routine. By using select_queue for this drivers
> need to be updated for each and every traffic type and we
> loose the goodness of much of the upstream work. For example
> txq caching.
> 
> Firmware solutions are inherently inflexible. And finally if
> admins are expected to build a qdisc and filter rules to steer
> traffic this requires knowledge of how the hardware is currently
> configured. The number of tx queues and the queue offsets may
> change depending on resources. Also this approach incurs all the
> overhead of a qdisc with filters.
> 
> With this mechanism users can set skb priority using expected
> methods either socket options or the stack can set this directly.
> Then the skb will be steered to the correct tx queues aligned
> with hardware QOS traffic classes. In the normal case with a
> single traffic class and all queues in this class every thing
> works as is until the LLD enables multiple tcs.
> 
> To steer the skb we mask out the lower 8 bits of the priority
> and allow the hardware to configure upto 15 distinct classes
> of traffic. This is expected to be sufficient for most applications
> at any rate it is more then the 8021Q spec designates and is
> equal to the number of prio bands currently implemented in
> the default qdisc.
> 
> This in conjunction with a userspace application such as
> lldpad can be used to implement 8021Q transmission selection
> algorithms one of these algorithms being the extended transmission
> selection algorithm currently being used for DCB.
> 
> If this approach seems reasonable I'll go ahead and finish
> this up. The priority to tc mapping should probably be exposed
> to userspace either through sysfs or rtnetlink. Any thoughts?
> 
> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
> ---
> 
>  include/linux/netdevice.h |   47 +++++++++++++++++++++++++++++++++++++++++++++
>  net/core/dev.c            |   43 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 89 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index b45c1b8..8a2adeb 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1092,6 +1092,12 @@ struct net_device {
>  	/* Data Center Bridging netlink ops */
>  	const struct dcbnl_rtnl_ops *dcbnl_ops;
>  #endif
> +	u8 max_tcs;
> +	u8 num_tcs;
> +	unsigned int *_tc_txqcount;
> +	unsigned int *_tc_txqoffset;

This seems wrong to use two different pointers, this is a waste of cache
memory. Also, I am not sure we need 32 bits, I believe we have a 16bit
limit for queue numbers.

Use a struct {
	u16 count;
	u16 offset;
};

> +	u64 prio_tc_map;

Seems wrong too on 32bit arches

	Please use : (even if using 16 bytes instead of 8)

	u8 prio_tc_map[16];

> +
>  
>  #if defined(CONFIG_FCOE) || defined(CONFIG_FCOE_MODULE)
>  	/* max exchange id for FCoE LRO by ddp */
> @@ -1108,6 +1114,44 @@ struct net_device {
>  #define	NETDEV_ALIGN		32
>  
>  static inline
> +int netdev_get_prio_tc_map(const struct net_device *dev, u32 prio)
> +{
> +	return (dev->prio_tc_map >> (4 * (prio & 0xF))) & 0xF;

	return dev->prio_tc_map[prio & 15];

> +}
> +
> +static inline
> +void netdev_set_prio_tc_map(struct net_device *dev, u8 prio, u8 tc)
> +{
> +	u64 mask = ~(-1 & (0xF << (4 * prio)));
> +	/* Zero the 4 bit prio map and set traffic class */
> +	dev->prio_tc_map &= mask;
> +	dev->prio_tc_map |= tc << (4 * prio);

	dev->prio_tc_map[prio & 15] = tc & 15;

> +}
> +
> +static inline
> +void netdev_set_tc_queue(struct net_device *dev, u8 tc, u16 count, u16 offset)
> +{
> +	dev->_tc_txqcount[tc] = count;
> +	dev->_tc_txqoffset[tc] = offset;
> +}
> +
> +static inline
> +int netdev_set_num_tc(struct net_device *dev, u8 num_tc)
> +{
> +	if (num_tc > dev->max_tcs)
> +		return -EINVAL;
> +
> +	dev->num_tcs = num_tc;
> +	return 0;
> +}
> +
> +static inline
> +u8 netdev_get_num_tc(struct net_device *dev)
> +{
> +	return dev->num_tcs;
> +}
> +
> +static inline
>  struct netdev_queue *netdev_get_tx_queue(const struct net_device *dev,
>  					 unsigned int index)
>  {
> @@ -1332,6 +1376,9 @@ static inline void unregister_netdevice(struct net_device *dev)
>  	unregister_netdevice_queue(dev, NULL);
>  }
>  
> +extern int		netdev_alloc_max_tcs(struct net_device *dev, u8 tcs);
> +extern void		netdev_free_tcs(struct net_device *dev);
> +
>  extern int 		netdev_refcnt_read(const struct net_device *dev);
>  extern void		free_netdev(struct net_device *dev);
>  extern void		synchronize_net(void);
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 4a587b3..4565afc 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2111,6 +2111,8 @@ static u32 hashrnd __read_mostly;
>  u16 skb_tx_hash(const struct net_device *dev, const struct sk_buff *skb)
>  {
>  	u32 hash;
> +	u16 qoffset = 0;
> +	u16 qcount = dev->real_num_tx_queues;
>  
>  	if (skb_rx_queue_recorded(skb)) {
>  		hash = skb_get_rx_queue(skb);
> @@ -2119,13 +2121,20 @@ u16 skb_tx_hash(const struct net_device *dev, const struct sk_buff *skb)
>  		return hash;
>  	}
>  
> +	if (dev->num_tcs) {
> +		u8 tc;
> +		tc = netdev_get_prio_tc_map(dev, skb->priority);
> +		qoffset = dev->_tc_txqoffset[tc];
> +		qcount = dev->_tc_txqcount[tc];
	
	Here, two cache lines accessed... with one pointer, only one cache
line.

> +	}
> +
>  	if (skb->sk && skb->sk->sk_hash)
>  		hash = skb->sk->sk_hash;
>  	else
>  		hash = (__force u16) skb->protocol ^ skb->rxhash;
>  	hash = jhash_1word(hash, hashrnd);
>  
> -	return (u16) (((u64) hash * dev->real_num_tx_queues) >> 32);
> +	return (u16) ((((u64) hash * qcount)) >> 32) + qoffset;
>  }
>  EXPORT_SYMBOL(skb_tx_hash);
>  
> @@ -5037,6 +5046,37 @@ void netif_stacked_transfer_operstate(const struct net_device *rootdev,
>  }
>  EXPORT_SYMBOL(netif_stacked_transfer_operstate);
>  
> +int netdev_alloc_max_tcs(struct net_device *dev, u8 tcs)
> +{
> +	unsigned int *count, *offset;
> +	count = kcalloc(tcs, sizeof(unsigned int), GFP_KERNEL);

for small tcs, you could get half a cache line, the other one might be
used elsewhere in the kernel, giving false sharing.

> +	if (!count)
> +		return -ENOMEM;
> +	offset = kcalloc(tcs, sizeof(unsigned int), GFP_KERNEL);

One allocation only ;)

> +	if (!offset) {
> +		kfree(count);
> +		return -ENOMEM;
> +	}
> +
> +	dev->_tc_txqcount = count;
> +	dev->_tc_txqoffset = offset;
> +	dev->max_tcs = tcs;
> +	return tcs;
> +}
> +EXPORT_SYMBOL(netdev_alloc_max_tcs);
> +
> +void netdev_free_tcs(struct net_device *dev)
> +{
> +	dev->max_tcs = 0;
> +	dev->num_tcs = 0;
> +	dev->prio_tc_map = 0;
> +	kfree(dev->_tc_txqcount);
> +	kfree(dev->_tc_txqoffset);
> +	dev->_tc_txqcount = NULL;
> +	dev->_tc_txqoffset = NULL;
> +}
> +EXPORT_SYMBOL(netdev_free_tcs);
> +
>  static int netif_alloc_rx_queues(struct net_device *dev)
>  {
>  #ifdef CONFIG_RPS
> @@ -5641,6 +5681,7 @@ void free_netdev(struct net_device *dev)
>  #ifdef CONFIG_RPS
>  	kfree(dev->_rx);
>  #endif
> +	netdev_free_tcs(dev);
>  
>  	kfree(rcu_dereference_raw(dev->ingress_queue));
>  
> 


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