netdev - Re: [RFC PATCH 2/2] ptr_ring_ll: pop/push multiple objects at once

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Date:   Tue, 15 Nov 2016 20:42:03 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     jasowang@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] ptr_ring_ll: pop/push multiple objects at once

On 16-11-14 03:06 PM, Michael S. Tsirkin wrote:
> On Thu, Nov 10, 2016 at 08:44:32PM -0800, John Fastabend wrote:
>> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
> 
> This will naturally reduce the cache line bounce
> costs, but so will a _many API for ptr-ring,
> doing lock-add many-unlock.
> 
> the number of atomics also scales better with the lock:
> one per push instead of one per queue.
> 
> Also, when can qdisc use a _many operation?
> 

On dequeue we can pull off many skbs instead of one at a time and
then either (a) pass them down as an array to the driver (I started
to write this on top of ixgbe and it seems like a win) or (b) pass
them one by one down to the driver and set the xmit_more bit correctly.

The pass one by one also seems like a win because we avoid the lock
per skb.

On enqueue qdisc side its a bit more evasive to start doing this.


[...]

>> +++ b/net/sched/sch_generic.c
>> @@ -571,7 +571,7 @@ static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc,
>>  	struct skb_array_ll *q = band2list(priv, band);
>>  	int err;
>>  
>> -	err = skb_array_ll_produce(q, skb);
>> +	err = skb_array_ll_produce(q, &skb);
>>  
>>  	if (unlikely(err)) {
>>  		net_warn_ratelimited("drop a packet from fast enqueue\n");
> 
> I don't see a pop many operation here.
> 

Patches need a bit of cleanup looks like it was part of another patch.

.John