bin/53285: Build times tripled with make/job.c 1.193

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         53285
>Category:       bin
>Synopsis:       Build times tripled with make/job.c 1.193
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun May 13 09:25:00 +0000 2018
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, source date >= 2018.05.12.18.17.04
>Organization:
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

The amount of time (real, not cpu) to build an amd64 relase on
babylon5.netbsd.org more than tripled with the commit of
src/usr.bin/make/job.c 1.193:

  babylon5.netbsd.org /bracket/i386/results/2018$ grep build_wall_time */bracket.db | tail -2
  2018.05.12.15.14.49/bracket.db:build_wall_time=4896.09
  2018.05.12.18.17.04/bracket.db:build_wall_time=16268.98

babylon5 is an 8-core, 16-hyperthread machine, building with -j 16.
The builds are niced and there is a few threads of other load, but I'm
also seeing a similar slowdown on my own build machine which has
no load other than the build.

Needless to say, this is negatively impacting the operation of the
TNF test infrastructure.

>How-To-Repeat:

Do a parallel release build on a multi-core machine with and without
make/job.c 1.193.  Compare build wall times.

>Fix:

Please revert the commit in case.