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

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Date:	Mon, 19 May 2014 07:09:47 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Wang <jasowang@...hat.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 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 ?

Is this -EIO thing a hard requirement, or a side effect ?

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.

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.

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.

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