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

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Message-ID: <20091005033256.GA26592@balbir.in.ibm.com>
Date:	Mon, 5 Oct 2009 09:02:56 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Len Brown <lenb@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-acpi@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Shaohua Li <shaohua.li@...el.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Subject: Re: [git pull request] ACPI Processor Aggregator Driver for
 2.6.32-rc1

* Len Brown <lenb@...nel.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@...el.com>
>     NACKed-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>     Cc: Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
>     Signed-off-by: Len Brown <len.brown@...el.com>

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

-- 
	Balbir
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