Re: [git pull request] ACPI Processor Aggregator Driver for 2.6.32-rc1

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Len Brown <lenb@kernel.org> [2009-10-03 01:56:32]:

>     This driver does not use the kernel's CPU hot-plug mechanism
>     because after the transient emergency is over, the system must
>     be returned to its normal state, and hotplug would permanently
>     break both cpusets and binding.
>     

Why does hotplug break cpusets and binding?


>     So to force idle, the driver creates a power saving thread.
>     The scheduler will migrate the thread to the preferred CPU.
>     The thread has max priority and has SCHED_RR policy,
>     so it can occupy one CPU.  To save power, the thread will
>     invoke the deep C-state entry instructions.
>     
>     To avoid starvation, the thread will sleep 5% of the time
>     time for every second (current RT scheduler has threshold
>     to avoid starvation, but if other CPUs are idle,
>     the CPU can borrow CPU timer from other,
>     which makes the mechanism not work here)
>     
>     Vaidyanathan Srinivasan has proposed scheduler enhancements
>     to allow injecting idle time into the system.  This driver doesn't
>     depend on those enhancements, but could cut over to them
>     when they are available.
>     
>     Peter Z. does not favor upstreaming this driver until
>     the those scheduler enhancements are in place.  However,
>     we favor upstreaming this driver now because it is useful
>     now, and can be enhanced over time.
>     
>     Signed-off-by: Shaohua Li <shaohua.li@intel.com>
>     NACKed-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
>     Cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
>     Signed-off-by: Len Brown <len.brown@intel.com>

This is a first a patch with a NACKed-by, could we please have more
discussion on the proposed design.

-- 
	Balbir