From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753442Ab2AaMsK (ORCPT ); Tue, 31 Jan 2012 07:48:10 -0500 Received: from casper.infradead.org ([85.118.1.10]:51088 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752243Ab2AaMsJ convert rfc822-to-8bit (ORCPT ); Tue, 31 Jan 2012 07:48:09 -0500 Message-ID: <1328014076.2446.210.camel@twins> Subject: Re: x86: clean up smpboot.c's use of udelay+schedule From: Peter Zijlstra To: Ingo Molnar Cc: Arjan van de Ven , linux-kernel@vger.kernel.org, x86@kernel.org, arjanvandeven@gmail.com, Thomas Gleixner Date: Tue, 31 Jan 2012 13:47:56 +0100 In-Reply-To: <20120131124341.GC4408@elte.hu> References: <20120130205304.0e10b05e@infradead.org> <20120131124341.GC4408@elte.hu> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.1- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2012-01-31 at 13:43 +0100, Ingo Molnar wrote: > * 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? wait_on_completion_timeout() and have the fresh cpu do a wakeup when its done. That way there's no need for a minimal wait time. Anyway, all the cpu hotplug code is a friggin trainwreck and needs a complete rewrite across all archs.