netdev - Re: [PATCH v4 0/10] bql: Byte Queue Limits

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Date:	Fri, 2 Dec 2011 15:22:28 +0400
From:	Kirill Smelkov <kirr@....spb.ru>
To:	David Miller <davem@...emloft.net>
Cc:	therbert@...gle.com, eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH v4 0/10] bql: Byte Queue Limits

On Thu, Dec 01, 2011 at 01:00:45PM -0500, David Miller wrote:
> From: Kirill Smelkov <kirr@....spb.ru>
> Date: Thu, 1 Dec 2011 20:50:18 +0400
> 
> > One "regression" is it is now not possible to disable BQL at compile time,
> > because CONFIG_BQL can't be set to "n" via usual ways.
> > 
> > Description and patch below. Thanks.
> 
> It's intentional, and your patch will not be applied.
> 
> The Kconfig entry and option is merely for expressing internal dependencies,
> not for providing a way to disable the code.

I'm maybe wrong somewhere - sorry then, but why there is e.g.

     static inline void netdev_tx_sent_queue(struct netdev_queue *dev_queue,
                                            unsigned int bytes)
     {
    +#ifdef CONFIG_BQL
    +       dql_queued(&dev_queue->dql, bytes);
    +       if (unlikely(dql_avail(&dev_queue->dql) < 0)) {
    +               set_bit(__QUEUE_STATE_STACK_XOFF, &dev_queue->state);
    +               if (unlikely(dql_avail(&dev_queue->dql) >= 0))
    +                       clear_bit(__QUEUE_STATE_STACK_XOFF,
    +                           &dev_queue->state);
    +       }
    +#endif
     }


and that netdev_tx_sent_queue() is called on every xmit.

I wanted to save cycles on my small/slow hardware and compile BQL out in
case I know the system is not going to use it. I'm netdev newcomer, so
sorry if I ask naive questions, but what's wrong it?


Thanks,
Kirill
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