MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <17505.39199.355967.509714@cargo.ozlabs.ibm.com>
Date: Wed, 10 May 2006 17:41:19 +1000
From: Paul Mackerras <paulus@samba.org>
To: "David S. Miller" <davem@davemloft.net>
Cc: olof@lixom.net, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
       linuxppc-dev@ozlabs.org
Subject: Re: [RFC/PATCH] Make powerpc64 use __thread for per-cpu variables
In-Reply-To: <20060509.233958.73723993.davem@davemloft.net>
References: <17505.26159.807484.477212@cargo.ozlabs.ibm.com>
	<20060510051649.GD1794@lixom.net>
	<17505.34919.750295.170941@cargo.ozlabs.ibm.com>
	<20060509.233958.73723993.davem@davemloft.net>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1019
Lines: 22

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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/