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

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Date:	Wed, 30 Aug 2006 14:54:12 +0200
From:	Andi Kleen <ak@...e.de>
To:	Chuck Ebbert <76306.1226@...puserve.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	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.

On Wednesday 30 August 2006 14:33, Chuck Ebbert wrote:
> In-Reply-To: <44F557A8.1030605@...p.org>
> 
> On Wed, 30 Aug 2006 02:17:28 -0700, Jeremy Fitzhardinge wrote:
> 
> > > This changes the ABI for signals and ptrace() and that seems like
> > > a bad idea to me.
> > >   
> > 
> > I don't believe it does; it certainly shouldn't change the usermode 
> > ABI.  How do you see it changing?
> 
> Nevermind.  I thought because you changed struct pt_regs in ptrace_abi.h
> it meant a user ABI change.

I think he broke the ptrace ABI actually in the first patch, but only by mistake 
and he promised to fix it :)

> 
> > > And the way things are done now is so ingrained into the i386
> > > kernel that I'm not sure it can be done.  E.g. I found two
> > > open-coded implementations of current, one in kernel_fpu_begin()
> > > and one in math_state_restore().

Perhaps those should be fixed? Is there a reason they are open coded?

> > >   
> > 
> > That's OK.  The current task will still be available in thread_info; 
> 
> But they can get out of sync, e.g. when switch_to() restores the new
> task's esp, the PDA still contains the old pcurrent and they don't get
> synchronized until the write_pda() in __switch_to().

But there is neither kernel_fpu_begin nor math_state_restore inbetween.
And I think interrupts are off too.

> 
> > To be honest, I haven't looked at percpu.h in great detail.  I was 
> > making assumptions about how it works, but it looks like they were wrong.
> 
> Would it make any sense to replace the 'cpu' field in thread_info with
> a pointer to a PDA-like structure?  We could even embed the static per_cpu
> data directly into that struct instead of chasing pointers...

I don't see what advantage it would have. %gs is clearly faster and shorter.

-Andi
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