netdev - Re: [PATCH] tcp: avoid wakeups for pure ACK

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Date:	Wed, 27 Feb 2013 10:17:39 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, ncardwell@...gle.com, therbert@...gle.com,
	ycheng@...gle.com, ak@...ux.intel.com
Subject: Re: [PATCH] tcp: avoid wakeups for pure ACK

On Wed, 2013-02-27 at 13:04 -0500, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Wed, 27 Feb 2013 09:05:03 -0800
> 
> > Processing pure ACK on behalf of the thread blocked in tcp_recvmsg()
> > is a waste of resources, as thread has to immediately sleep again
> > because it got no payload.
> 
> More than one thread can be operating on the socket, the other one
> could be waiting for the window to open up in order to do a send.  Are
> you absolutely sure that we won't have a problem in that situation?


Yes, more than one thread can be operating, but the prequeue wakeups the
one blocked in tcp_recvmsg() only, because of :

wake_up_interruptible_sync_poll(sk_sleep(sk),
				POLLIN | POLLRDNORM | POLLRDBAND);


Then the ACK processing might/should wakeup the other thread blocked in
tcp_sendmsg().

So this patch will also help this (not very usual) situation, as we will
only wakeup the tcp_sendmsg() thread when ACK is processed from softirq
handler, and let the thread blocked in tcp_recvmsg() sleeping.


> In fact I wonder if that does the right thing right now.

Right now it is working, because at least one thread will process the
prequeue at the exit of tcp_recvmsg()


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