Subject: Re: [RFC PATCH v2 01/10] net: sched: allow qdiscs to handle locking

From: John Fastabend <[email-masked]>
Date: Wed, 13 Jul 2016 23:44:55 -0700

On 16-07-13 11:19 PM, John Fastabend wrote:
> This patch adds a flag for queueing disciplines to indicate the stack
> does not need to use the qdisc lock to protect operations. This can
> be used to build lockless scheduling algorithms and improving
> performance.
> 
> The flag is checked in the tx path and the qdisc lock is only taken
> if it is not set. For now use a conditional if statement. Later we
> could be more aggressive if it proves worthwhile and use a static key
> or wrap this in a likely().
> 
> Signed-off-by: John Fastabend <[email-masked]>
> ---

[...]

> @@ -3075,6 +3075,27 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
>  	int rc;
>  
>  	qdisc_calculate_pkt_len(skb, q);
> +
> +	if (q->flags & TCQ_F_NOLOCK) {
> +		if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
> +			__qdisc_drop(skb, &to_free);
> +			rc = NET_XMIT_DROP;
> +		} else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q)) {
> +			qdisc_bstats_cpu_update(q, skb);
> +			__qdisc_run(q);

Reviewing these patches now and noticed this qdisc_run() is not
needed.

> +			if (sch_direct_xmit(skb, q, dev, txq, root_lock, true))
> +				__qdisc_run(q);
> +			rc = NET_XMIT_SUCCESS;
> +		} else {
> +			rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
> +			__qdisc_run(q);
> +		}
> +
> +		if (unlikely(to_free))
> +			kfree_skb_list(to_free);
> +		return rc;
> +	}
> +

[...]

Thanks,
John