Hello,

I currently have Ubuntu 14.04.01 LTS, Kernel 3.13.0-32 on a dual core laptop. I am working on creating a low latency kernel image of 3.12.74
version and preempt_rt patch.

While make menuconfig, (using the .config file as received while installing Ubuntu 14.04.1),
I see only 2 options under "Timer Subsystems"
-> Old Idle dynticks config
-> High resolution Timer support

--------------------------------------------------------------

It is missing option related to CONFIG_NO_HZ_FULL and other similar options.
It is not a problem of kernel supporting the feature, as it is available as per documentation from 3.10 kernel.

This is relvant excerpt from original config which am modifying:
#
# Timers subsystem
#
CONFIG_TICK_ONESHOT=y
CONFIG_NO_HZ_COMMON=y
# CONFIG_HZ_PERIODIC is not set
CONFIG_NO_HZ_IDLE=y
CONFIG_NO_HZ=y
CONFIG_HIGH_RES_TIMERS=y


(Not sure and dont believe I have disabled anything that would make this option disabled). So, now, I believe this particular config file which I received aling with Ubuntu 14 installation is incomplete. But am not an expert at these options, so I have 2 simple questions:
1) How do I modify config file (There might be dependencies etc and other things might need to be done, instead of just adding another macro in config file as CONFIG_NO_HZ_FULL)
2) How do I deal with this issue in general, as always I would be using some existing config file, which might be incomplete, and I might be missing some options which I might want to try out.

Thank you

OK, you need to get the config file from somewhere. Where did it come from? I usually use the latest /boot/config-xyz file, and just rename it .config. Then, before you can manually configure the kernel, you have to do 'make oldconfig'. Then you can do a menuconfig, or xconfig, or gconfig.

Dear AwesomeMachine,
I have used the .config file taken from existing installation and reused it. I did directly run make menuconfig.
I have by now tried multiple things, (also checked make help for a view of make targets of linux kernel), make defconfig and then make oldconfig and then make menuconfig, (or) make alldefconfig and then make menuconfig (or) make oldconfig and then make menuconfig etc various combinations of these but still I do not get the option corresponding to CONFIG_NO_HZ_FULL.
The documentation resources (for example ...https://lwn.net/Articles/549580/) show that the option has been present since 3.10 and am building 3.12.x version of kernel.

My question is straight forward, why am I unable to see the option. Any inputs towards resolution or anything that I misunderstood. I have actually searched in kernel source directory for CONFIG_NO_HZ_FULL but I did not get any results in config files but I got the macro in source and header files.Exact source version of kernel am trying to build is 3.12.74.

Thank you

I just realized that the default arch provided config file for 64 bit one (x86_64_defconfig) contains the option CONFIG_NO_HZ_FULL, the 32 bit variant(i386_defconfig) doesnt contain the macro.

"I am not sure if that is intentional and the 32 bit kernel 3.12 doesnt support CONFIG_NO_HZ_FULL option (or) it is something that needs to be fixed (or) overlooked to be updated".

1) So, any comments and inputs and detail on my understanding(above statement) is greatly appreciated.

2)Can I manually edit the 32 bit config file to include the macro? (It din work, I tried already) (or) should I try to compare the 64 bit config file and 32 bit one and copy relevant changes to the 32 bit one (Am about to give this a try now)

Thank you very much

32-bit processors probably don't use that config option, or it's always set the same, so there is no option to change it on a 32-bit kernel.

I noticed you are not running 'make clean' at all. After you configure the kernel, before you configure it again, you have to run 'make clean'.