netdev - Re: [PATCH v2] net-tun: restructure tun_do_read for better sleep/wakeup efficiency

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Date:	Tue, 20 May 2014 14:03:34 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Xi Wang <xii@...gle.com>, "David S. Miller" <davem@...emloft.net>,
	netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
	Maxim Krasnyansky <maxk@....qualcomm.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH v2] net-tun: restructure tun_do_read for better sleep/wakeup
 efficiency

On 05/20/2014 01:11 PM, Eric Dumazet wrote:
> On Tue, 2014-05-20 at 12:44 +0800, Jason Wang wrote:
>> On 05/19/2014 10:09 PM, Eric Dumazet wrote:
>>> About the sk_data_ready() and wake_up_all(), you missed the whole part
>>> of the patch I think.
>>>
>>> Check how sock_def_readable() does everything properly and efficiently,
>>> including the async part.
>> But this changes (sk_data_ready()) has nothing related to switching to
>> use __skb_recv_datagram()
>>
> This is totally related.
>
> I think you did not yet understood this patch

Sorry for being unclear, but I think you misunderstand my meaning.
>
> Compare wake_up_all() and sk_data_ready() speeds, you'll be surprised.
>
> You should ask to yourself : Why do we use wq_has_sleeper() in
> networking stacks ?

See my first reply, I don't have objection that uses sk_data_ready() in
tun_net_xmit(). My only concern is using sk_data_ready() in
tun_detach_all():

- It was only called during tun destroying, so I believe we don't care
about the performance in this condition.
- sk_data_ready() was usually called when there's something new to be
processed which is not case in tun_detach_all()
- Not sure it was a problem but sock_def_readable() will not wake up
uninterruptible task during tun destroying.
- If we make sock_fasync() works for tun in the future, it may send
SIGIO to user process during tun destroying which is not expected.

Thanks
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