How to set intel_idle.max_cstate=0 to disable c-states? - linux-kernel

Question

I would like to disable c-states on my computer.
I disabled c-state on BIOS but I don't obtain any result. However, I found an explanation :
"Most newer Linux distributions, on systems with Intel processors, use the “intel_idle” driver (probably compiled into your kernel and not a separate module) to use C-states. This driver uses knowledge of the various CPUs to control C-states without input from system firmware (BIOS). This driver will mostly ignore any other BIOS setting and kernel parameters"
I found two solutions to solve this problem but I don't know how to apply:
1) " so if you want control over C-states, you should use kernel parameter “intel_idle.max_cstate=0” to disable this driver."
I don't know neither how I can check the value (of intel_idle.max_cstate ) and neither how I can change its value.
2) "To dynamically control C-states, open the file /dev/cpu_dma_latency and write the maximum allowable latency to it. This will prevent C-states with transition latencies higher than the specified value from being used, as long as the file /dev/cpu_dma_latency is kept open. Writing a maximum allowable latency of 0 will keep the processors in C0"
I can't read the file cpu_dma_latency.
Thanks for your help.
Computer:
Intel Xeon CPU E5-2620
Gnome 2.28.2
Linux 2.6.32-358

Answer 1

To alter the value at boot time, you can modify the GRUB configuration or edit it on the fly -- the method to modify that varies by distribution. This is the Ubuntu documentation to change kernel parameters either for a single boot, or permanently. For a RHEL-derived distribution, I don't see docs that are quite as clear, but you directly modify /boot/grub/grub.conf to include the parameter on the "kernel" lines for each bootable stanza.
For the second part of the question, many device files are read-only or write-only. You could use a small perl script like this (untested and not very clean, but should work) to keep the file open:
#!/usr/bin/perl
use FileHandle;
my $fd = open (">/dev/cpu_dma_latency");
print $fd "0";
print "Press CTRL-C to end.\n";
while (1) {
sleep 5;
}
Redhat has a C snippet in a KB article here as well and more description of the parameter.