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

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Date:	Tue, 20 May 2014 12:44:01 +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 05/19/2014 10:09 PM, Eric Dumazet wrote:
> On Mon, 2014-05-19 at 17:27 +0800, Jason Wang wrote:
>
>> Still a little bit difference. We check the reg_state after we're sure
>> there's nothing left in sk_receive_queue. But this patch returns -EIO
>> before trying to dequeue skb.
> Do you think its a problem, other than a simple difference of behavior ?

 I just point out the possible issues.
>
> Is this -EIO thing a hard requirement, or a side effect ?

Probably not, but it's really not hard to keep this.
>
> Either we try to converge things, or we keep this driver a pile of
> copy/pasted stuff from other locations, missing improvements we did
> in core networking stack, because of some supposed differences.

Agree, but we'd better separate the unrelated changes into other patches
to simplify review and speed up convergence.
>
> 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()

The async part may not work since tun has its own implementation of fasync.
>
> static void sock_def_readable(struct sock *sk)
> {
>         struct socket_wq *wq;
>
>         rcu_read_lock();
>         wq = rcu_dereference(sk->sk_wq);
>         if (wq_has_sleeper(wq))
>                 wake_up_interruptible_sync_poll(&wq->wait, POLLIN | POLLPRI |
>                                                 POLLRDNORM | POLLRDBAND);
>         sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
>         rcu_read_unlock();
> }
>
> Using sk_data_ready() is the way to reach sock_def_readable()
> as its not an EXPORT_SYMBOL.
>
> If this driver was using the common interface, we would not have
> these false sharing problems, that were solved a long time ago.

Yes, but better with a separate patch.

We can probably rework the tun_fasync() to use sock_fasync(), but using
sk_data_ready() in tun_detach_all() is still questionable. We may not
want to send EIO to userspace when tun device is destroying.
>
>
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