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SubjectRe: x86: clean up smpboot.c's use of udelay+schedule

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

> >From 9f8dd2b15ff19ad73ee0eb235b4fdde9277185e8 Mon Sep 17 00:00:00 2001
> From: Arjan van de Ven <arjan@linux.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@linux.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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