From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754200Ab2AaMny (ORCPT ); Tue, 31 Jan 2012 07:43:54 -0500 Received: from mx2.mail.elte.hu ([157.181.151.9]:49204 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753024Ab2AaMnx (ORCPT ); Tue, 31 Jan 2012 07:43:53 -0500 Date: Tue, 31 Jan 2012 13:43:41 +0100 From: Ingo Molnar To: Arjan van de Ven Cc: linux-kernel@vger.kernel.org, x86@kernel.org, arjanvandeven@gmail.com, Thomas Gleixner , Peter Zijlstra Subject: Re: x86: clean up smpboot.c's use of udelay+schedule Message-ID: <20120131124341.GC4408@elte.hu> References: <20120130205304.0e10b05e@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120130205304.0e10b05e@infradead.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.3.1 -2.0 BAYES_00 BODY: Bayes spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Arjan van de Ven wrote: > >From 9f8dd2b15ff19ad73ee0eb235b4fdde9277185e8 Mon Sep 17 00:00:00 2001 > From: Arjan van de Ven > Date: Mon, 30 Jan 2012 20:38:09 -0800 > Subject: [PATCH] x86: clean up smpboot.c's use of udelay+schedule > > smpboot.c does a udelay() followed by a schedule(); to yield > the CPU to other threads. This comes from the time when the kernel > did not yet have usleep_*() style APIs... > > ... nowadays, the kernel can do better than this, and this > patch replaces this code sequence with a usleep_range(), > so that the CPU is actually yielded for some real time. > > Signed-off-by: Arjan van de Ven > --- > arch/x86/kernel/smpboot.c | 3 +-- > 1 files changed, 1 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c > index 66d250c..0b794c6 100644 > --- a/arch/x86/kernel/smpboot.c > +++ b/arch/x86/kernel/smpboot.c > @@ -781,14 +781,13 @@ do_rest: > for (timeout = 0; timeout < 50000; timeout++) { > if (cpumask_test_cpu(cpu, cpu_callin_mask)) > break; /* It has booted */ > - udelay(100); > /* > * Allow other tasks to run while we wait for the > * AP to come online. This also gives a chance > * for the MTRR work(triggered by the AP coming online) > * to be completed in the stop machine context. > */ > - schedule(); > + usleep_range(100, 200); I'm wondering whether we could shorten this delay to say 10 usecs and thus save 0.1 msecs (or more) from a typical SMP bootup? Thanks, Ingo