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

Mathieu Desnoyers
2010-01-07T15:08:06Z

* Peter Zijlstra (peterz@infradead.org) wrote:
> 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.
>

I like this ! :)

Following some testing, I think I'll go with your scheme, with 2
smp_call_function_single (one function call for the local thread, one
IPI). If we need more than that, then we allocate a cpumask and call
smp_call_function_many() for the other cpus. I provide benchmarks in my
reply to Josh justifying this choice.

Thanks,

Mathieu

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Mathieu Desnoyers
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