netdev - Re: [RFC PATCH 05/12] net: sched: per cpu gso handlers

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Date:	Wed, 30 Dec 2015 12:42:09 -0800
From:	John Fastabend <john.fastabend@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
CC:	daniel@...earbox.net, eric.dumazet@...il.com, jhs@...atatu.com,
	aduyck@...antis.com, davem@...emloft.net,
	john.r.fastabend@...el.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH 05/12] net: sched: per cpu gso handlers

On 15-12-30 12:26 PM, Jesper Dangaard Brouer wrote:
> On Wed, 30 Dec 2015 09:52:49 -0800
> John Fastabend <john.fastabend@...il.com> wrote:
> 
>> The net sched infrastructure has a gso ptr that points to skb structs
>> that have failed to be enqueued by the device driver.
> 
> What about fixing up the naming "gso" to something else like "requeue",
> in the process (or by an pre-patch) ?

Sure I'll throw a patch in front of this to rename it.

> 
> 
>> This can happen when multiple cores try to push a skb onto the same
>> underlying hardware queue resulting in lock contention. This case is
>> handled by a cpu collision handler handle_dev_cpu_collision(). Another
>> case occurs when the stack overruns the drivers low level tx queues
>> capacity. Ideally these should be a rare occurrence in a well-tuned
>> system but they do happen.
>>
>> To handle this in the lockless case use a per cpu gso field to park
>> the skb until the conflict can be resolved. Note at this point the
>> skb has already been popped off the qdisc so it has to be handled
>> by the infrastructure.
> 
> I generally like this idea of resolving this per cpu.  (I stalled here,
> on the requeue issue, last time I implemented a lockless qdisc
> approach).
> 

Great, this approach seems to work OK.

On another note even if we only get a single skb dequeued at a time in
the initial implementation this is still a win as soon as we start
running classifiers/actions. Even if doing simple pfifo_fast sans
classifiers raw throughput net gain is minimal.

.John
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