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

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Date:	Sun, 27 Aug 2006 18:01:44 +0200
From:	Andi Kleen <ak@...e.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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.


Very cool.

> - Make it work.  It works UP on a test QEMU machine, but it doesn't
>   yet work on real hardware, or SMP (though not working SMP on QEMU is
>   more likely to be a QEMU problem).  Not sure what the problem is yet;
>   I'm hoping review will reveal something.

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?

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

> - 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.

> - 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.

(weak point:)

- The stack protector code might work one day on i386 too.

-Andi
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