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]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=200709281934.11957.ak@suse.de \
--to=ak@suse.de \
--cc=linux-arch@vger.kernel.org \
--cc=ralf@linux-mips.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).