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

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 11 Apr 2009 20:06:05 -0400 Dave Jones <davej@redhat.com> wrote:

> On Fri, Apr 10, 2009 at 11:17:18PM -0700, Andrew Morton wrote:
>  > From: Andrew Morton <akpm@linux-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.
> 
> Wouldn't it be a better to make smp_call_function_single do this check
> itself, so all callers benefit from this optimisation?
> 
> *looks*
> 
> Wait, won't this already be caught by this code in smp_call_function_single() ?
> 
> 286         this_cpu = get_cpu();
> ...
> 291         if (cpu == this_cpu) {
> 292                 local_irq_save(flags);
> 293                 func(info);
> 294                 local_irq_restore(flags);
> 295         } else {
> 
> 
> 

The problem is that the caller (acpi-cpufreq) is doing

	cpu = cpumask_any(mask);
	smp_call_function_single(cpu);

and cpumask_any(mask) does cpumask_first(mask).  Which might be a
different CPU, even though this thread of control is running on a CPU
which is present in `mask'.

- We could fix this by making cpumask_any(mask) return this-cpu if
  this-cpu is present `mask'.

- We could fix this by changing smp_call_function_single() to take a
  mask, rather than a particular CPU.  Then of course it preferentially
  chooses this-cpu if possible.

  Or write a new smp_call_function_any(mask, ...);

I suspect that changing cpumask_any() to preferentially return this-cpu
will always give us the behaviour that we prefer, but I haven't looked
into it.