tcp_splice SO_RCVLOWAT code; never invoked?

[David Gibson <david@gibson.dropbear.id.au>]

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Hi Stefano,

When debugging the splice EINTR bug I fixed the other day, I found the
whole tcp_splice_sock_handler() pretty confusing to follow.  So, I was
working on some cleanups.  But then I noticed something more
specifically odd here.

We've discussed the use of SO_RCVLOWAT previously.  AIUI, you found it
essential to achieve reasonable throughput and load for spliced
connections.  I think we've agreed before that it's not entirely the
right tool for the job; just the only one available.

Except... as far as I can tell, it's never invoked.  AFAICT the only
place we enable the RCVLOWAT stuff is in a block under this if:

	if (!(conn->flags & lowat_set_flag) &&
	    readlen > (long)c->tcp.pipe_size / 10) {

But... this occurs immediately after:
	if (readlen >= (long)c->tcp.pipe_size * 10 / 100)
		continue;

.. which is a strictly more inclusive condition, so we'll never reach
the RCVLOWAT block.

To confirm, I tried putting an ASSERT(0) in that block, and didn't hit
it with spliced iperf3 runs.

Am I missing something?

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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