CONFIG_NO_HZ_FULL restricts cpu usage to the equivalent of one in 4.2

[Ken Moffat <zarniwhoop@ntlworld.com>]

New title, I originally posted this last night but I've now made
a little progress in identifying what changed.  Previous thread was
labelled for AMD Phenom, but it is more general.  CC'ing Jeff
because he replied to the original, I guess he probably won't be
interested after this.

Yesterday was the first day I had done any real compiling with
4.2-rc.  I started out using -rc1, and make -j4 on a recent LFS/BLFS
system (gcc-5.1.0, make-4.1, etc).  I wanted to start trying to
build kde5, and I expected a lot of issues.  What I did not expect
was that qt5 would build as if I was using -j1.

Examination eventually identified that with 4.2-rc1 and 4.2-rc2,
make ran the number of jobs I had specified, but the total of the
CPU percentages ('top' from procps-ng-3.3.10) maxed out at 100%.  On
4.1 kernels the percentage with -j4 maxes out at 400% (my machine
has 4 cores).  I suspected either an unfortunate choice in 'make
oldconfig', or something specific to an AMD Phenom / gcc-5.1.

Today I have tried make -j4 on two other machines with 4.2-rc
kernels [ building the current git stable release ].  On my i3
SandyBridge everything was fine, CPU usage approached 400%.  On my
AMD A10-7850K I have the same problem as on the phenom.  Not
surprising, I began by using the Phenom config when I got the A10,
then adapted it to suit, whereas the i3 has much less memory so I
haven't made many changes since I got it.

Comparing the configs for the i3 and the A10, the first thing which
looked as if it might be relevant was the _CPU_ACCOUNTING choices.
Those on the A10 seem to be driven from CONFIG_NO_HZ_FULL so I began
by changing that to CONFIG_NO_HZ_IDLE.  Payday ;-)  make -j4 now
approaches 400% CPU usage.

The config differences follow.  Perhaps it is actually one of the
subsequent choices that is the problem.  And I guess it could still
be a gcc-5.1 issue.

--- config-4.2-initial	2015-07-15 16:25:12.548005751 +0100
+++ config-4.2-speed-ok	2015-07-15 17:00:50.919998703 +0100
@@ -104,11 +104,8 @@
 CONFIG_TICK_ONESHOT=y
 CONFIG_NO_HZ_COMMON=y
 # CONFIG_HZ_PERIODIC is not set
-# CONFIG_NO_HZ_IDLE is not set
-CONFIG_NO_HZ_FULL=y
-CONFIG_NO_HZ_FULL_ALL=y
-CONFIG_NO_HZ_FULL_SYSIDLE=y
-CONFIG_NO_HZ_FULL_SYSIDLE_SMALL=4
+CONFIG_NO_HZ_IDLE=y
+# CONFIG_NO_HZ_FULL is not set
 # CONFIG_NO_HZ is not set
 CONFIG_HIGH_RES_TIMERS=y
 
@@ -116,7 +113,9 @@
 # CPU/Task time and stats accounting
 #
 CONFIG_VIRT_CPU_ACCOUNTING=y
+# CONFIG_TICK_CPU_ACCOUNTING is not set
 CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
+# CONFIG_IRQ_TIME_ACCOUNTING is not set
 # CONFIG_BSD_PROCESS_ACCT is not set
 CONFIG_TASKSTATS=y
 CONFIG_TASK_DELAY_ACCT=y
@@ -131,7 +130,6 @@
 # CONFIG_TASKS_RCU is not set
 CONFIG_RCU_STALL_COMMON=y
 CONFIG_CONTEXT_TRACKING=y
-CONFIG_RCU_USER_QS=y
 CONFIG_CONTEXT_TRACKING_FORCE=y
 # CONFIG_TREE_RCU_TRACE is not set
 CONFIG_RCU_NOCB_CPU=y

Anyway, I'll start a bisection.  But it might take me a few days,
this is not a convenient time (somehow, kernel issues which need
bisection always come at a bad time for me).

ĸen
-- 
This one goes up to eleven!