selftests: net: avoid ptl lock contention in tcp_mmap

tcp_mmap is used as a reference program for TCP rx zerocopy,
so it is important to point out some potential issues.

If multiple threads are concurrently using getsockopt(...
TCP_ZEROCOPY_RECEIVE), there is a chance the low-level mm
functions compete on shared ptl lock, if vma are arbitrary placed.

Instead of letting the mm layer place the chunks back to back,
this patch enforces an alignment so that each thread uses
a different ptl lock.

Performance measured on a 100 Gbit NIC, with 8 tcp_mmap clients
launched at the same time :

$ for f in {1..8}; do ./tcp_mmap -H 2002:a05:6608:290:: & done

In the following run, we reproduce the old behavior by requesting no alignment :

$ tcp_mmap -sz -C $((128*1024)) -a 4096
received 32768 MB (100 % mmap'ed) in 9.69532 s, 28.3516 Gbit
  cpu usage user:0.08634 sys:3.86258, 120.511 usec per MB, 171839 c-switches
received 32768 MB (100 % mmap'ed) in 25.4719 s, 10.7914 Gbit
  cpu usage user:0.055268 sys:21.5633, 659.745 usec per MB, 9065 c-switches
received 32768 MB (100 % mmap'ed) in 28.5419 s, 9.63069 Gbit
  cpu usage user:0.057401 sys:23.8761, 730.392 usec per MB, 14987 c-switches
received 32768 MB (100 % mmap'ed) in 28.655 s, 9.59268 Gbit
  cpu usage user:0.059689 sys:23.8087, 728.406 usec per MB, 18509 c-switches
received 32768 MB (100 % mmap'ed) in 28.7808 s, 9.55074 Gbit
  cpu usage user:0.066042 sys:23.4632, 718.056 usec per MB, 24702 c-switches
received 32768 MB (100 % mmap'ed) in 28.8259 s, 9.5358 Gbit
  cpu usage user:0.056547 sys:23.6628, 723.858 usec per MB, 23518 c-switches
received 32768 MB (100 % mmap'ed) in 28.8808 s, 9.51767 Gbit
  cpu usage user:0.059357 sys:23.8515, 729.703 usec per MB, 14691 c-switches
received 32768 MB (100 % mmap'ed) in 28.8879 s, 9.51534 Gbit
  cpu usage user:0.047115 sys:23.7349, 725.769 usec per MB, 21773 c-switches

New behavior (automatic alignment based on Hugepagesize),
we can see the system overhead being dramatically reduced.

$ tcp_mmap -sz -C $((128*1024))
received 32768 MB (100 % mmap'ed) in 13.5339 s, 20.3103 Gbit
  cpu usage user:0.122644 sys:3.4125, 107.884 usec per MB, 168567 c-switches
received 32768 MB (100 % mmap'ed) in 16.0335 s, 17.1439 Gbit
  cpu usage user:0.132428 sys:3.55752, 112.608 usec per MB, 188557 c-switches
received 32768 MB (100 % mmap'ed) in 17.5506 s, 15.6621 Gbit
  cpu usage user:0.155405 sys:3.24889, 103.891 usec per MB, 226652 c-switches
received 32768 MB (100 % mmap'ed) in 19.1924 s, 14.3222 Gbit
  cpu usage user:0.135352 sys:3.35583, 106.542 usec per MB, 207404 c-switches
received 32768 MB (100 % mmap'ed) in 22.3649 s, 12.2906 Gbit
  cpu usage user:0.142429 sys:3.53187, 112.131 usec per MB, 250225 c-switches
received 32768 MB (100 % mmap'ed) in 22.5336 s, 12.1986 Gbit
  cpu usage user:0.140654 sys:3.61971, 114.757 usec per MB, 253754 c-switches
received 32768 MB (100 % mmap'ed) in 22.5483 s, 12.1906 Gbit
  cpu usage user:0.134035 sys:3.55952, 112.718 usec per MB, 252997 c-switches
received 32768 MB (100 % mmap'ed) in 22.6442 s, 12.139 Gbit
  cpu usage user:0.126173 sys:3.71251, 117.147 usec per MB, 253728 c-switches

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Arjun Roy <arjunroy@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>