linux-kernel - Re: [PATCH RFC 0/6] Implement per-processor data areas for i386.

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Date:	Sun, 27 Aug 2006 09:41:17 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <ak@...e.de>
CC:	linux-kernel@...r.kernel.org,
	Chuck Ebbert <76306.1226@...puserve.com>,
	Zachary Amsden <zach@...are.com>,
	Jan Beulich <jbeulich@...ell.com>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH RFC 0/6] Implement per-processor data areas for i386.

Andi Kleen wrote:
> I bet qemu doesn't have a real descriptor cache unlike real CPUs.
> So likely it is some disconnect between changing the backing GDT
> and referencing the register. Reload %gs more aggressively?
>   

The GDT only gets touched once in cpu_init(), and %gs is reloaded on 
every kernel entry, so I don't think that's it.  I seems to have 
interrupt issues with SMP.

And either way, it still doesn't work on real hardware...

> Comparing with SimNow! (which should behave more like a real CPU)
> might be also interesting.
>   

Yeah, I'll have to try that out.

>> - Measure performance impact.  The patch adds a segment register
>>   save/restore on entry/exit to the kernel.  This expense should be
>>   offset by savings in using the PDA while in the kernel, but I haven't
>>   measured this yet.  Space savings are already appealing though.
>> - Modify more things to use the PDA.  The more that uses it, the more
>>   the cost of the %gs save/restore is amortized.  smp_processor_id and
>>   current are the obvious first choices, which are implemented in this
>>   series.
>>     
>
> per cpu data would be the prime candidate. It is pretty simple.
>   

Well, it has to be arch-specific per-cpu data, since the PDA is arch 
specific.  But there should be various pieces of interrupt state that 
adapt well to it.

>> - Make it a config option?  UP systems don't need to do any of this,
>>   other than having a single pre-allocated PDA.  Unfortunately, it gets
>>   a bit messy to do this given the changes needed in handling %gs.
>>     
>
> Please don't.
>   

Yeah, that wasn't really a serious thought...

    J
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