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Date: Wed, 10 May 2006 17:41:19 +1000
From: Paul Mackerras <paulus@samba.org>
To: "David S. Miller" <davem@davemloft.net>
Subject: Re: [RFC/PATCH] Make powerpc64 use __thread for per-cpu variables
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David S. Miller writes:

> That first cache line of current_thread_info() should be so hot that
> it's probably just fine to use current_thread_info()->task since
> you're just doing a mask on a fixed register (r1) to implement that.

I tried that, but I found that adding 1 instruction to the sequence
for getting current adds about 8k to the kernel text.  Currently we do
it in one instruction, that would be two - the mask and the load.  It
probably doesn't make a measurable difference to performance, but it
doesn't look good.  The number of instructions we lose by using
__thread is much less than the 8k we gain from using
current_thread_info()->task for current.  So I'd prefer to use a
per-cpu variable for current, since we can get to that in 1
instruction.

Paul.