From: Andi Kleen <ak@suse.de>
To: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-arch@vger.kernel.org
Subject: Re: Optimize cpumask functions for SMPs with < BITS_PER_LONG processors
Date: Fri, 28 Sep 2007 19:34:11 +0200 [thread overview]
Message-ID: <200709281934.11957.ak@suse.de> (raw)
In-Reply-To: <20070925155200.GA7342@linux-mips.org>
On Tuesday 25 September 2007 17:52:00 Ralf Baechle wrote:
> When debugging a kernel using a logic analyzer (!) a colleague recently
> noticed that because the <linux/cpumasks.h> functions are based on the
> generic bitops which support arbitrary size bitfields we had a relativly
> high overhead resulting from this. Here's the chainsaw edition of a patch
> to optimize this for CONFIG_NR_CPUS <= BITS_PER_LONG. Comments?
The right thing to test is not CONFIG_NR_CPUS, but just
do __builtin_constant_p(x) && (x) <= BITS_PER_LONG ? fast case : external call
in find_*_bit()
x86-64 has done this already for some time. But one issue is that
that the cpumask walk functions currently do (n = find_*_bit()) >= maxbit ? maxbit : n
which also creates more overhead because some architectures get this
wrong (including x86-64 I must admit)
-Andi
prev parent reply other threads:[~2007-09-28 17:34 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-09-25 15:52 Optimize cpumask functions for SMPs with < BITS_PER_LONG processors Ralf Baechle
2007-09-28 17:34 ` Andi Kleen [this message]
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