Re: [PATCH] cpu-freq: add troubleshooting section for FSB changes

[Thomas Renninger <trenn@suse.de>]

On Friday 06 November 2009 09:01:00 pm Luis R. Rodriguez wrote:
> With cpu frequency scaling you may run into some system
> performance issues when the FSB changes speed due to the
> throughput constraints this brings on the system.
Interesting.
Could you be a bit more detailed how and what kind of perfomance issues are
seen, please.
I expect IO based issues like WLAN package throughput or even lost connections :)
"wireless" is also a good word to grep for...
If this gets documented also a bit more info about the HW should get
added. The problem sounds rather restricted to specific models (atom based, laptop,
desktop chipsets?)?
Otherwise everyone adding a 10G network card into his Intel machine will
start measuring or excluding the lowest freq when reading this.

Thanks for the valuable info,

      Thomas

> Document how to rule out whether or not FSB changes
> affect general system issues.
>
> Cc: Matthew Garrett <mjg@redhat.com>
> Cc: Arjan van de Ven" <arjan@linux.intel.com>
> Cc: Reinette Chatre <reinette.chatre@intel.com>
> Cc: Aeolus Yang <Aeolus.Yang@Atheros.com>
> Cc: Amod Bodas <Amod.Bodas@Atheros.com>
> Cc: David Quan <David.Quan@Atheros.com>
> Cc: Kishore Jotwani <Kishore.Jotwani@Atheros.com>
> Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
> ---
>  Documentation/cpu-freq/user-guide.txt |   45
> +++++++++++++++++++++++++++++++++ 1 files changed, 45 insertions(+), 0
> deletions(-)
>
> diff --git a/Documentation/cpu-freq/user-guide.txt
> b/Documentation/cpu-freq/user-guide.txt index 2a5b850..846729d 100644
> --- a/Documentation/cpu-freq/user-guide.txt
> +++ b/Documentation/cpu-freq/user-guide.txt
> @@ -32,6 +32,7 @@ Contents:
>  3. How to change the CPU cpufreq policy and/or speed
>  3.1 Preferred interface: sysfs
>
> +4. Troubleshooting
>
>
>  1. Supported Architectures and Processors
> @@ -211,3 +212,47 @@ scaling_setspeed.		By "echoing" a new frequency into
> this you can change the speed of the CPU,
>  				but only within the limits of
>  				scaling_min_freq and scaling_max_freq.
> +
> +4. Troubleshooting
> +==================
> +
> +If you are not sure if frequency scaling is the culprit to your
> +issues you can try to disable the feature on the BIOS if possible.
> +
> +Alternatively it helps to understand that on some CPUs (Intel)
> +the Front Side Bus (FSB) speed is changed when you happen to change
> +the active CPU frequency to the lowest supported frequency, which
> +happens to be the frequency used on the highest supported P-n state.
> +The P0 state is the highest Performance state with the highest
> +CPU frequency and system power. The highest P-n state would be
> +the one with the lowest supported frequency and power.
> +
> +On Intel CPUs the FSB will not be modified with SpeedStep enabled if the
> +second to last lowest frequency on the available scaling frequencies
> +is used. Testing this would rule out whether or not the FSB changes
> +made by SpeedStep would be the main issue.
> +
> +Modifying the FSB will alter your general system throughput
> +on your Northbridge so to prevent general throughput issues
> +with devices on it you can modify frequency scaling so that
> +it nevers uses the lowest supported frequency.
> +
> +For example:
> +
> +cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_frequencies
> +1801000 1800000 1200000 800000
> +1801000 1800000 1200000 800000
> +
> +cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_min_freq
> +800000
> +800000
> +
> +To disable changing the FSB select the second to last CPU freq
> +on the available freq for each CPU and set the scaling_min_freq
> +for each CPU to that value. For the example output this would be
> +echo'ing 1200000 to each scaling_min_freq:
> +
> +echo 1200000 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq
> +echo 1200000 > /sys/devices/system/cpu/cpu1/cpufreq/scaling_min_freq
> +
> +The first line is for the 1st CPU, the second line for the second line.