On Sat, 11 Apr 2009 20:06:05 -0400 Dave Jones <davej@xxxxxxxxxx> wrote:
> On Fri, Apr 10, 2009 at 11:17:18PM -0700, Andrew Morton wrote:
> > From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >
> > 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.
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