kern/55063: kernel iscsi is flaky

[tnn%nygren.pp.se@localhost]

>Number:         55063
>Category:       kern
>Synopsis:       kernel iscsi is flaky
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 11 13:15:00 +0000 2020
>Originator:     Tobias Nygren
>Release:        4.99.49
>Organization:
>Environment:
amd64
>Description:
Using the kernel scsi initiator against iscsi-target with 4kB block size I frequently run into some sort of resource limit:

sd0: QUEUE FULL resulted in 0 openings

At first the messages are occasional but then it starts spamming and 
the kernel hard hangs after a few minutes. Cannot enter ddb at this point.

>How-To-Repeat:
host 1: run /usr/sbin/iscsi-target -b 4096
host 2: run iscsid(8), newfs /dev/dk0 with "-O2 -f 4096 -b 32768" and mount with -o log,noatime,nodevtime. Apply moderate I/O like untar or anoncvs checkout for 2-3 minutes.


>Fix:
Tried bumping CCBS_PER_SESSION, CCBS_FOR_SCSIPI, PDUS_PER_CONNECTION but it didn't help.