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

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Date:	Mon, 5 Oct 2009 21:59:24 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	balbir@...ux.vnet.ibm.com
Cc:	Len Brown <lenb@...nel.org>,
	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

On Monday 05 October 2009, Balbir Singh wrote:
> * 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.

This thing has already been merged, it appears:

commit 8e0af5141ab950b78b3ebbfaded5439dcf8b3a8d
Author: Shaohua Li <shaohua.li@...el.com>
Date:   Mon Jul 27 18:11:02 2009 -0400

    ACPI: create Processor Aggregator Device driver

and it looks like a total breakage of rules to me.

Thanks,
Rafael
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