I see the cores on an Intel i5 machine I'm looking at can only be run at the same clockspeed: /sys/devices/system/cpu/cpu1/cpufreq/related_cpus lists all of the CPUs. Setting cpu1's clockspeed changes cpu0's, as expected.

Supposedly the AMD A6-4400M machine I'm running should be able to run each core at a different clockspeed:/sys/devices/system/cpu/cpu1/cpufreq/related_cpu only lists cpu1. When I set cpu1's clockspeed by using the performance governor and echoing 1400000 to scaling_max_freq, cpu0's clockspeed remains at 2700000 as expected. Cpu1's scaling_cur_freq reads 1400000 as expected.

However, cpu1's cpuinfo_cur_freq reads 2700000. From benchmarking, it appears CPU1 is indeed still running at 2.7 GHz. Am I missing something, or is something broken?

I'm running Linux 2.6.35, and passing idle=mwait in the kernel command line.


This is not yet close to be a definite answer. Instead, it's a set of suggestions too long to fit in comments.

I'm afraid you might slightly misinterpret the meanings of sysfs cpufreq parameters. For instance, on my Core Duo laptop, the related_cpu parameters for both cores read 0 1 - which, according to your interpretation, would mean that the cores cannot switch frequencies independently. But that is not the case - I can set each frequency at will. By contrast, the affected_cpus parameter for each core lists only the respective CPU number.

You might want to take a look at kernel documentation for cpu-freq to get a better understanding of the parameters such as affected_cpus,related_cpus,scaling_* and cpuinfo_*. The documentation is normally distributed with kernel source packages. Specifically, I recommend reading <kernel-sources-dir>/Documentation/cpu-freq/user-guide.txt, where <kernel-sources-dir> would typically stand for /usr/src/linux or /usr/src/linux-<kernel-version>. (However, when I skim through the documentation myself now, I confess I don't catch some of the frequency-scaling-related nuances. To fully understand these, one probably needs to gain a solid understanding of CPU architectures first.)

Back to your question. And one more test case on my part: when I change the value of scaling_max_freq (with either userspace or performance governor being used), the core's clock automatically switches to that new maximum. The different behaviour you're observing might be any of:

  • specific to hardware implementation of frequency scaling mechanisms on your CPU,
  • due to differences between the standard cpufreq module and phc-intel which I'm using,
  • normal behaviour (call it a bug or a feature if you will) of cpufreq module, which has changed at some point since 2.6.35 (my current kernel version is 3.6.2),
  • result of a bug in cpufreq implementation for your CPU (or entire family),
  • specific to the implementation of performance CPU governor as of 2.6.35.

Some of the things you might do to push your investigation further:

  • read the user-guide.txt and fiddle more with other cpufreq parameters,
  • repeat the tests while running a newer kernel - the easiest way is to boot a liveCD/DVD/USB.

If you continue to experience unexpected behaviour and gain more reasons to believe it is due to a bug (definitely must check with the latest minor kernel version), go ahead and report this on kernel.org bugzilla.

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