netdev - Re: [PATCH] tcp: allow splice() to build full TSO packets

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Message-Id: <20120403.173614.962252876842659412.davem@davemloft.net>
Date:	Tue, 03 Apr 2012 17:36:14 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, ncardwell@...gle.com, therbert@...gle.com,
	ycheng@...gle.com, hkchu@...gle.com, maze@...gle.com,
	maheshb@...gle.com, ilpo.jarvinen@...sinki.fi, nanditad@...gle.com
Subject: Re: [PATCH] tcp: allow splice() to build full TSO packets

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 03 Apr 2012 23:31:29 +0200

> The code in tcp_sendmsg() and do_tcp_sendpages() is similar (actually
> probably copy/pasted) but the thing is tcp_sendmsg() is called once per
> sendmsg() call (and the push logic is OK at the end of it), while a
> single splice() system call can call do_tcp_sendpages() 16 times (or
> even more if pipe buffer was extended by fcntl(F_SETPIPE_SZ))

Ok, so this means that in essence the tcp_mark_push should also only
be done in the final sendpage call.

And since I'm wholly convinced that the URG stuff is a complete
"don't care" for this path, I'm convinced your patch is the right
thing to do.

Applied to 'net' and queued up for -stable, thanks Eric.

> Maybe a real fix would be to call do_tcp_sendpages() exactly once, but I
> tried this today and found needed surgery was complex). Also this would
> lock socket for a long period and could add latencies because of backlog
> processing.

I don't think this is a good idea.  Maybe we can do some level of
batching at some point, but it would need to have a limit.

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