netdev - Re: [PATCH 3/9 Rev3] [sched] Modify qdisc

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Date:	Wed, 8 Aug 2007 16:14:02 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Krishna Kumar <krkumar2@...ibm.com>
Cc:	davem@...emloft.net, shemminger@...ux-foundation.org,
	kaber@...sh.net, sri@...ibm.com, jagana@...ibm.com,
	Robert.Olsson@...a.slu.se, peter.p.waskiewicz.jr@...el.com,
	herbert@...dor.apana.org.au, gaagaan@...il.com,
	kumarkr@...ux.ibm.com, rdreier@...co.com, rick.jones2@...com,
	mcarlson@...adcom.com, jeff@...zik.org,
	general@...ts.openfabrics.org, mchan@...adcom.com, tgraf@...g.ch,
	hadi@...erus.ca, netdev@...r.kernel.org, xma@...ibm.com
Subject: Re: [PATCH 3/9 Rev3] [sched] Modify qdisc_run to support batching

On Wed, Aug 08, 2007 at 03:01:45PM +0530, Krishna Kumar (krkumar2@...ibm.com) wrote:
> +static inline int get_skb(struct net_device *dev, struct Qdisc *q,
> +			  struct sk_buff_head *blist, struct sk_buff **skbp)
> +{
> +	if (likely(!blist || (!skb_queue_len(blist) && qdisc_qlen(q) <= 1))) {
> +		return likely((*skbp = dev_dequeue_skb(dev, q)) != NULL);
> +	} else {
> +		int max = dev->tx_queue_len - skb_queue_len(blist);
> +		struct sk_buff *skb;
> +
> +		while (max > 0 && (skb = dev_dequeue_skb(dev, q)) != NULL)
> +			max -= dev_add_skb_to_blist(skb, dev);
> +
> +		*skbp = NULL;
> +		return 1;	/* we have atleast one skb in blist */
> +	}
> +}

Same here - is it possible to get a list in one go instead of pulling
one-by-one, since it forces quite a few additional unneded lock
get/releases. What about dev_dequeue_number_skb(dev, q, num), which will 
grab the lock and move a list of skbs from one queue to provided head.

> @@ -158,7 +198,10 @@ static inline int qdisc_restart(struct n
>  	/* And release queue */
>  	spin_unlock(&dev->queue_lock);
>  
> -	ret = dev_hard_start_xmit(skb, dev);
> +	if (likely(skb))
> +		ret = dev_hard_start_xmit(skb, dev);
> +	else
> +		ret = dev->hard_start_xmit_batch(dev);

Perfectionism says that having array of two functions and calling one of
them via array_func_pointer[!!skb] will be much faster. Just a though.
It is actually much faster than if/else on x86 at least.

-- 
	Evgeniy Polyakov
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