linux-kernel - Re: x86: clean up smpboot.c's use of udelay+schedule

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Date:	Tue, 31 Jan 2012 13:43:41 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	arjanvandeven@...il.com, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: x86: clean up smpboot.c's use of udelay+schedule


* Arjan van de Ven <arjan@...radead.org> wrote:

> >From 9f8dd2b15ff19ad73ee0eb235b4fdde9277185e8 Mon Sep 17 00:00:00 2001
> From: Arjan van de Ven <arjan@...ux.intel.com>
> 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 <arjan@...ux.intel.com>
> ---
>  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
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