Re: [RFC PATCH] thermal: add generic cpu hotplug cooling device

[Hongbo Zhang <hongbo.zhang@freescale.com>]

On 09/21/2013 06:15 AM, Zoran Markovic wrote:
> This patch implements a generic CPU hotplug cooling device. The
> implementation scales down the number of running CPUs when temperature
> increases through a thermal trip point and prevents booting CPUs
> until thermal conditions are restored. Upon restoration, the action
> of starting up a CPU is left to another entity (e.g. CPU offline
> governor, for which a patch is in the works).
>
> In the past two years, ARM considerably reduced the time required for
> CPUs to boot and shutdown; this time is now measured in microseconds.
> This patch is predominantly intended for ARM big.LITTLE architectures
> where big cores are expected to have a much bigger impact on thermal
> budget than little cores, resulting in fast temperature ramps to a trip
> point, i.e. thermal runaways. Switching off the big core(s) may be one
> of the recovery mechanisms to restore system temperature, but the actual
> strategy is left to the thermal governor.
>
> The assumption is that CPU shutdown/startup is a rare event, so no
> attempt was made to make the code atomic, i.e. the code evidently races
> with CPU hotplug driver. The set_cur_state() function offlines CPUs
> iteratively one at a time, checking the cooling state before each CPU
> shutdown. A hotplug notifier callback validates any CPU boot requests
> against current cooling state and approves/denies accordingly. This
> mechanism guarantees that the desired cooling state could be reached in a
> maximum of d-c iterations, where d and c are the "desired" and "current"
> cooling states expressed in the number of offline CPUs.
>
> Credits to Amit Daniel Kachhap for initial attempt to upstream this feature.
>
> Cc: Zhang Rui<rui.zhang@intel.com>
> Cc: Eduardo Valentin<eduardo.valentin@ti.com>
> Cc: Rob Landley<rob@landley.net>
> Cc: Amit Daniel Kachhap<amit.daniel@samsung.com>
> Cc: Andrew Morton<akpm@linux-foundation.org>
> Cc: Durgadoss R<durgadoss.r@intel.com>
> Cc: Christian Daudt<bcm@fixthebug.org>
> Cc: James King<james.king@linaro.org>
> Signed-off-by: Zoran Markovic<zoran.markovic@linaro.org>
> ---
>   Documentation/thermal/cpu-cooling-api.txt |   17 ++
>   drivers/thermal/Kconfig                   |   10 +
>   drivers/thermal/Makefile                  |    1 +
>   drivers/thermal/cpu_hotplug.c             |  362 +++++++++++++++++++++++++++++
>   include/linux/cpuhp_cooling.h             |   57 +++++
>   5 files changed, 447 insertions(+)
>   create mode 100644 drivers/thermal/cpu_hotplug.c
>   create mode 100644 include/linux/cpuhp_cooling.h
Only form my point of view:
I like the name of cpu_hotplug_cooling.c and cpu_hotplug_cooling.h
we already have a cpu_cooling.c, that isn't so exact either because we 
have more than one method to cool a CPU, these c and h files should be 
renamed to cpu_freq_cooling.c and cpu_freq_cooling.h later.

By the way, some servers with tens of CPUs may need this patch too.