[#LU-13712] Flaw in MR Routing Algorithm

While testing the fix for ~~LU-13708~~, we found that communication was still severely disrupted.

I ran some ping tests between a client and a server. From the logs, we could see the router was attempting to forward a message using an interface that had been disabled:

00000400:00000200:0.0:1593005930.168691:0:7991:0:(lib-move.c:4323:lnet_parse()) TRACE: 10.16.100.56@o2ib10(605@gni) <- 610@gni : GET - routed
00000800:00000200:0.0:1593005930.168698:0:7991:0:(gnilnd_cb.c:2450:kgnilnd_recv()) $$ conn ffff88082b37c800, rxmsg ffffc900201321c8, lntmsg ffff880716ef6040 niov=0 kiov=          (null) iov=          (null) offset=0 mlen=0 rlen=0 from 610@gni  msg@0xffffc900201321c8 m/v/ty/ck/pck/pl b00fbabe/8/2/0/0/0 x2156869:GNILND_MSG_IMMEDIATE
00000800:00000200:0.0:1593005930.168707:0:7991:0:(gnilnd_cb.c:2092:kgnilnd_consume_rx()) $$ rx ffff880829491d80 processed from 610@gni  msg@0xffffc900201321c8 m/v/ty/ck/pck/pl b00fbabe/8/2/0/0/0 x2156869:GNILND_MSG_IMMEDIATE
00000800:00000200:0.0:1593005930.168711:0:7991:0:(gnilnd_cb.c:2058:kgnilnd_release_msg()) consuming ffff88082b37c800
00000400:00000200:0.0:1593005930.168718:0:7991:0:(lib-msg.c:996:lnet_is_health_check()) health check = 1, status = 0, hstatus = 0
00000400:00000200:0.0:1593005930.168722:0:7991:0:(lib-msg.c:825:lnet_health_check()) health check: 605@gni->610@gni: GET: OK
00000400:00000200:0.0:1593005930.168727:0:7991:0:(lib-move.c:2624:lnet_handle_send_case_locked()) Source ANY to NMR:  10.16.100.56@o2ib10 local destination
00000400:00000200:0.0:1593005930.168737:0:7991:0:(lib-move.c:1853:lnet_handle_send()) TRACE: 610@gni(10.16.100.14@o2ib10:<?>) -> 10.16.100.56@o2ib10(10.16.100.56@o2ib10:10.16.100.56@o2ib10) <?> : GET try# 0 <<<< 10.16.100.14@o2ib10 is disabled interface

There's a flaw in the logic used to forward the message. The path selection code treats this like a local send to a non-multi-rail peer. The reason for this is that we don't want the router to modify the destination interface. However, this code path sets a "preferred NI" that gets used for future sends. In this case, the first time the router forwarded a message to 10.16.100.56@o2ib10, it set 10.16.100.14@o2ib10 as the "preferred NI" to be used when communicating with this node. Now, even though that interface is down, it is still selecting it to forward messages because of this preferred status.

[LU-13712] Flaw in MR Routing Algorithm Created: 24/Jun/20 Updated: 07/Aug/20 Resolved: 07/Aug/20
Status:	Resolved
Project:	Lustre
Component/s:	None
Affects Version/s:	None
Fix Version/s:	Lustre 2.14.0

[LU-13712] Flaw in MR Routing Algorithm Created: 24/Jun/20 Updated: 07/Aug/20 Resolved: 07/Aug/20