[SRU][B/aws, F/aws, G/aws][PATCH 0/1] aws: fix network performance regression due to initial TCP buffer size change

[Andrea Righi]

BugLink: https://bugs.launchpad.net/bugs/1910200

[Impact]

AWS has seen some customers reporting networking performance degradation
after they upgraded their Ubuntu instanceses. This regression is highly
impacting customers who are using MTU=9000 (which is the default in
EC2).

[Test case]

Bug reproduced internally in AWS (no test case provided), but apparently
it is very easy to reproduce simply by measuring networking performance.

[Fix]

AWS worked internally and found that this regression has been introduced
by:

 a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB")

To solve this problem we need to apply the following upstream commit
that explicitly fixes the problem introduced by the commit above:

 33ae7b5bb841 ("tcp: select sane initial rcvq_space.space for big MSS")

[Regression potential]

Upstream fix that is only affecting the initial TCP buffer space and
allows the TCP window size to be dynamically increased, basically
restoring the previous (correct) behavior, so regression potential is
minimal.