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Date: Sun, 16 Apr 2006 17:02:19 +1000
From: Paul Mackerras <paulus@samba.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>, LKML <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@osdl.org>, Linus Torvalds <torvalds@osdl.org>,
       Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
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       lethal@linux-sh.org, Chris Zankel <chris@zankel.net>,
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       parisc-linux@parisc-linux.org, linuxppc-dev@ozlabs.org,
       linux390@de.ibm.com, davem@davemloft.net, rusty@rustcorp.com.au
Subject: Re: [PATCH 00/05] robust per_cpu allocation for modules
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Steven Rostedt writes:

> So now I'm asking for advice on some ideas that can be a work around to
> keep the robustness and speed.

Ideally, what I'd like to do on powerpc is to dedicate one register to
storing a per-cpu base address or offset, and be able to resolve the
offset at link time, so that per-cpu variable accesses just become a
register + offset memory access.  (For modules, "link time" would be
module load time.)

We *might* be able to use some of the infrastructure that was put into
gcc and binutils to support TLS (thread local storage) to achieve
this.  (See http://people.redhat.com/drepper/tls.pdf for some of the
details of that.)

Also, I've added Rusty Russell to the cc list, since he designed the
per-cpu variable stuff in the first place, and would be able to
explain the trade-offs that led to the PERCPU_ENOUGH_ROOM thing.  (I
think you're discovering them as you go, though. :)

Paul.