linux-kernel - Re: [patch for 2.6.30 2/2] arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c: avoid cross-CPU interrupts

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Date:	Sun, 19 Apr 2009 22:57:24 -0400 (EDT)
From:	Len Brown <lenb@...nel.org>
To:	akpm@...ux-foundation.org
Cc:	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	davej@...hat.com, efault@....de, len.brown@...el.com,
	mingo@...e.hu, rusty@...tcorp.com.au, tglx@...utronix.de,
	venkatesh.pallipadi@...el.com, yakui.zhao@...el.com,
	yanmin_zhang@...ux.intel.com
Subject: Re: [patch for 2.6.30 2/2] arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c:
 avoid cross-CPU interrupts



> From: Andrew Morton <akpm@...ux-foundation.org>
> 
> In drv_read(), check to see whether we can run the rdmsr() on the current
> CPU.  If so, do that.  So smp_call_function_single() can avoid the IPI.
> 
> Arguably, cpumask_any() should do this.
> 
> Cc: Rusty Russell <rusty@...tcorp.com.au>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
> Cc: Len Brown <len.brown@...el.com>
> Cc: Zhao Yakui <yakui.zhao@...el.com>
> Cc: Dave Jones <davej@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Tested-by: Mike Galbraith <efault@....de>
> Cc: "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> 
> ---
> 
>  arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c |   17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff -puN arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c~arch-x86-kernel-cpu-cpufreq-acpi-cpufreqc-avoid-cross-cpu-interrupts arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c
> --- a/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c~arch-x86-kernel-cpu-cpufreq-acpi-cpufreqc-avoid-cross-cpu-interrupts
> +++ a/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c
> @@ -197,9 +197,22 @@ static void do_drv_write(void *_cmd)
>  
>  static void drv_read(struct drv_cmd *cmd)
>  {
> -	cmd->val = 0;
> +	int target_cpu;		/* The CPU on which to perform thr rdmsr() */
> +	int this_cpu;
> +
> +	/*
> +	 * If the current CPU is in cmd->mask then run the rdmsr() on this
> +	 * CPU to avoid the cross-cpu interrupt.
> +	 */
> +	this_cpu = get_cpu();
> +	if (cpu_isset(this_cpu, *(cmd->mask)))
> +		target_cpu = this_cpu;
> +	else
> +		target_cpu = cpumask_any(cmd->mask);
>  
> -	smp_call_function_single(cpumask_any(cmd->mask), do_drv_read, cmd, 1);
> +	cmd->val = 0;
> +	smp_call_function_single(target_cpu, do_drv_read, cmd, 1);
> +	put_cpu();
>  }
>  
>  static void drv_write(struct drv_cmd *cmd)
> _

Rather than this patch I would expect we would want to either:

A. as we went to the trouble to detect the local case
   in drv_read, why call smp_call_function at all for that case?

or

B. optimize smp_call_function_single to beneift all users
   instead of just this customer.

thanks,
-Len Brown, Intel Open Source Technology Center

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