Re: [RFC PATCH] introduce sys_membarrier(): process-wide memorybarrier

[Peter Zijlstra]

On Wed, 2010-01-06 at 22:35 -0800, Josh Triplett wrote:
> 
> The number of threads doesn't matter nearly as much as the number of
> threads typically running at a time compared to the number of
> processors.  Of course, we can't measure that as easily, but I don't
> know that your proposed heuristic would approximate it well.

Quite agreed, and not disturbing RT tasks is even more important.

A simple:

  for_each_cpu(cpu, current->mm->cpu_vm_mask) {
     if (cpu_curr(cpu)->mm == current->mm)
        smp_call_function_single(cpu, func, NULL, 1);
  }

seems far preferable over anything else, if you really want you can use
a cpumask to copy cpu_vm_mask in and unset bits and use the mask with
smp_call_function_any(), but that includes having to allocate the
cpumask, which might or might not be too expensive for Mathieu.



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