Message-Id: <201208052334.15253.rjw@sisk.pl>
Date:	Sun, 5 Aug 2012 23:34:15 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Andre Przywara <andre.przywara@....com>
Cc:	cpufreq@...r.kernel.org, Matthew Garrett <mjg@...hat.com>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Thomas Renninger <trenn@...e.de>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/8] Documentation: Add documentation for boost control switch

On Thursday, July 26, 2012, Andre Przywara wrote:
> The new acpi-cpufreq driver supports a system global control switch
> to disable the frequency boosting feature of some (x86) CPUs.
> Provide documentation about the rationale and the usage.
> 
> Signed-off-by: Andre Przywara <andre.przywara@....com>

That should be folded into the patch adding the feature being documented.

Thanks,
Rafael


> ---
>  Documentation/ABI/testing/sysfs-devices-system-cpu |   12 +++
>  Documentation/cpu-freq/boost.txt                   |   94 ++++++++++++++++++++
>  2 files changed, 106 insertions(+), 0 deletions(-)
>  create mode 100644 Documentation/cpu-freq/boost.txt
> 
> diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
> index 5dab364..1107a2e 100644
> --- a/Documentation/ABI/testing/sysfs-devices-system-cpu
> +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
> @@ -176,3 +176,15 @@ Description:	Disable L3 cache indices
>  		All AMD processors with L3 caches provide this functionality.
>  		For details, see BKDGs at
>  		http://developer.amd.com/documentation/guides/Pages/default.aspx
> +
> +
> +What:		/sys/devices/system/cpu/cpufreq/boost
> +Date:		August 2012
> +Contact:	Linux kernel mailing list <linux-kernel@...r.kernel.org>
> +Description:	Processor frequency boosting control
> +
> +		This switch controls the boost setting for the whole system.
> +		Boosting allows the CPU and the firmware to run at a frequency
> +		beyound it's nominal limit.
> +		More details can be found in Documentation/cpu-freq/boost.txt
> +
> diff --git a/Documentation/cpu-freq/boost.txt b/Documentation/cpu-freq/boost.txt
> new file mode 100644
> index 0000000..0085494
> --- /dev/null
> +++ b/Documentation/cpu-freq/boost.txt
> @@ -0,0 +1,94 @@
> +Processor boosting control
> +
> +	- information for users -
> +
> +Quick guide for the impatient:
> +--------------------
> +/sys/devices/system/cpu/cpufreq/boost
> +controls the boost setting for the whole system. You can read and write
> +that file with either "0" (boosting disabled) or "1" (boosting allowed).
> +Reading or writing 1 does not mean that the system is boosting at this
> +very moment, but only that the CPU _may_ raise the frequency at it's
> +discretion.
> +--------------------
> +
> +Introduction
> +-------------
> +Some CPUs support a functionality to raise the operating frequency of
> +some cores in a multi-core package if certain conditions apply, mostly
> +if the whole chip is not fully utilized and below it's intended thermal
> +budget. This is done without operating system control by a combination
> +of hardware and firmware.
> +On Intel CPUs this is called "Turbo Boost", AMD calls it "Turbo-Core",
> +in technical documentation "Core performance boost". In Linux we use
> +the term "boost" for convenience.
> +
> +Rationale for disable switch
> +----------------------------
> +
> +Though the idea is to just give better performance without any user
> +intervention, sometimes the need arises to disable this functionality.
> +Most systems offer a switch in the (BIOS) firmware to disable the
> +functionality at all, but a more fine-grained and dynamic control would
> +be desirable:
> +1. While running benchmarks, reproducible results are important. Since
> +   the boosting functionality depends on the load of the whole package,
> +   single thread performance can vary. By explicitly disabling the boost
> +   functionality at least for the benchmark's run-time the system will run
> +   at a fixed frequency and results are reproducible again.
> +2. To examine the impact of the boosting functionality it is helpful
> +   to do tests with and without boosting.
> +3. Boosting means overclocking the processor, though under controlled
> +   conditions. By raising the frequency and the voltage the processor
> +   will consume more power than without the boosting, which may be
> +   undesirable for instance for mobile users. Disabling boosting may
> +   save power here, though this depends on the workload.
> +
> +
> +User controlled switch
> +----------------------
> +
> +To allow the user to toggle the boosting functionality, the acpi-cpufreq
> +driver exports a sysfs knob to disable it. There is a file:
> +/sys/devices/system/cpu/cpufreq/boost
> +which can either read "0" (boosting disabled) or "1" (boosting enabled).
> +Reading the file is always supported, even if the processor does not
> +support boosting. In this case the file will be read-only and always
> +reads as "0". Explicitly changing the permissions and writing to that
> +file anyway will return EINVAL.
> +
> +On supported CPUs one can write either a "0" or a "1" into this file.
> +This will either disable the boost functionality on all cores in the
> +whole system (0) or will allow the hardware to boost at will (1).
> +
> +Writing a "1" does not explicitly boost the system, but just allows the
> +CPU (and the firmware) to boost at their discretion. Some implementations
> +take external factors like the chip's temperature into account, so 
> +boosting once does not necessarily mean that it will occur every time
> +even using the exact same software setup.
> +
> +
> +AMD legacy cpb switch
> +---------------------
> +The AMD powernow-k8 driver used to support a very similar switch to
> +disable or enable the "Core Performance Boost" feature of some AMD CPUs.
> +This switch was instantiated in each CPU's cpufreq directory
> +(/sys/devices/system/cpu[0-9]*/cpufreq) and was called "cpb".
> +Though the per CPU existence hints at a more fine grained control, the
> +actual implementation only supported a system-global switch semantics,
> +which was simply reflected into each CPU's file. Writing a 0 or 1 into it
> +would pull the other CPUs to the same state.
> +For compatibility reasons this file and its behavior is still supported
> +on AMD CPUs, though it is now protected by a config switch
> +(X86_ACPI_CPUFREQ_CPB). On Intel CPUs this file will never be created,
> +even with the config option set.
> +This functionality is considered legacy and will be removed in some future
> +kernel version.
> +
> +More fine grained boosting control
> +----------------------------------
> +
> +Technically it is possible to switch the boosting functionality at least
> +on a per package basis, for some CPUs even per core. Currently the driver
> +does not support it, but this may be implemented in the future.
> +
> 

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