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

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Message-ID: <20151230212633.23fc9b7c@redhat.com>
Date:	Wed, 30 Dec 2015 21:26:33 +0100
From:	Jesper Dangaard Brouer <brouer@...hat.com>
To:	John Fastabend <john.fastabend@...il.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,
	brouer@...hat.com
Subject: Re: [RFC PATCH 05/12] net: sched: per cpu gso handlers

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) ?


> 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).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer
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