netdev - Re: [RFC PATCH 01/13] net: sched: allow qdiscs to handle locking

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Date:	Wed, 17 Aug 2016 15:48:50 -0700
From:	John Fastabend <john.fastabend@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	xiyou.wangcong@...il.com, jhs@...atatu.com,
	alexei.starovoitov@...il.com, brouer@...hat.com,
	john.r.fastabend@...el.com, netdev@...r.kernel.org,
	davem@...emloft.net
Subject: Re: [RFC PATCH 01/13] net: sched: allow qdiscs to handle locking

On 16-08-17 03:34 PM, Eric Dumazet wrote:
> On Wed, 2016-08-17 at 12:33 -0700, John Fastabend wrote:
> 
> 
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 4ce07dc..5db395d 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -3076,6 +3076,26 @@ 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)) {
> 
> For a lockless qdisc, do you believe TCQ_F_CAN_BYPASS is still a gain ?
> 

For the benchmarks from pktgen it appears to be a win or mute to just
drop the TCQ_F_CAN_BYPASS (just taking a look at one sample below)

    nolock & nobypass   locked (current master)
----------------------------------------------
1:  1435796 		1471479
2:  1880642		1746231
4:  1922935		1119626
8:  1585055		1001471
12: 1479273		989269

The only thing would be to test a bunch of netperf RR sessions to be
sure.

> Also !qdisc_qlen(q) looks racy anyway ?

Yep its racy unless you make it an atomic but this hurts performance
metrics. There is a patch further in the stack here that adds the
atomic variants but I tend to think we can just drop the bypass logic
in the lockless case assuming the netperf tests look good.

> 
>> +			qdisc_bstats_cpu_update(q, skb);
>> +			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;
>> +	}
>> +
> 
>