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

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Date:	Wed, 13 Jul 2016 23:44:55 -0700
From:	John Fastabend <john.fastabend@...il.com>
To:	fw@...len.de, jhs@...atatu.com, alexei.starovoitov@...il.com,
	eric.dumazet@...il.com, brouer@...hat.com
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC PATCH v2 01/10] net: sched: allow qdiscs to handle locking

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 <john.r.fastabend@...el.com>
> ---

[...]

> @@ -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