Date: Tue, 31 May 2005 08:34:28 -0600 (MDT)
From: Zwane Mwaikambo <zwane@arm.linux.org.uk>
To: Srivatsa Vaddagiri <vatsa@in.ibm.com>
cc: jayush luniya <jayu_11@yahoo.com>, linux-kernel@vger.kernel.org
Subject: Re: HOTPLUG CPU Support for SMT
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On Tue, 31 May 2005, Srivatsa Vaddagiri wrote:

> On Mon, May 30, 2005 at 04:50:27PM +0000, Zwane Mwaikambo wrote:
> > Yes, older 2.6-mm kernel (2.6.10-mm) trees have the "toy" i386 hotplug 
> > cpu implementation which does what you want.
> 
> AFAIK in the i386 "toy" implementation, when a CPU is offlined, it stops
> taking interrupts and stops running tasks, but it _still_ executes a while
> loop in the context of its idle task (with IRQs disabled). The loop
> is exited when we have to bring online the CPU again. What this means is 
> I don't think by offlining the CPU, we are removing any activity associated
> with the corresponding h/w thread. 
> 
> Maybe the toy implementation could be modified to take care of it? Something
> like lowering the priority of the h/w thread so that it consumes minimal 
> CPU resources to execute its while loop.

A cpu_relax() there would help greatly and essentially "drops" the 
priority of the processor executing it (since 'pause' is a hint that that 
logical processor would like to yield execution resources), although there 
would still be traffic due to accessing get_cpu_var, which should be minimal as 
it'll be at most 1 non shared cacheline so you could account for it in the 
statement of errors. I used to have a personal implementation for one of 
the systems i use which does a hlt in that loop and i wake up the 
procsesor via IPI. So perhaps some kernel hacking might be required 
(albeit minimal) ;)

	Zwane

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