Re: Udpated sys_membarrier() speedup patch, FYI

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 28, 2017, at 1:31 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:

> On Fri, Jul 28, 2017 at 10:15:49AM -0700, Andrew Hunter wrote:
>> On Thu, Jul 27, 2017 at 12:43 PM, Paul E. McKenney
>> <paulmck@linux.vnet.ibm.com> wrote:
>> > On Thu, Jul 27, 2017 at 10:20:14PM +0300, Avi Kivity wrote:
>> >> IPIing only running threads of my process would be perfect. In fact
>> >> I might even be able to make use of "membarrier these threads
>> >> please" to reduce IPIs, when I change the topology from fully
>> >> connected to something more sparse, on larger machines.
>> 
>> We do this as well--sometimes we only need RSEQ fences against
>> specific CPU(s), and thus pass a subset.
> 
> Sounds like a good future enhancement, probably requiring a new syscall
> to accommodate the cpumask.
> 
>> > +static void membarrier_private_expedited_ipi_each(void)
>> > +{
>> > +       int cpu;
>> > +
>> > +       for_each_online_cpu(cpu) {
>> > +               struct task_struct *p;
>> > +
>> > +               rcu_read_lock();
>> > +               p = task_rcu_dereference(&cpu_rq(cpu)->curr);
>> > +               if (p && p->mm == current->mm)
>> > +                       smp_call_function_single(cpu, ipi_mb, NULL, 1);
>> > +               rcu_read_unlock();
>> > +       }
>> > +}
>> > +
>> 
>> We have the (simpler imho)
>> 
>> const struct cpumask *mask = mm_cpumask(mm);
>> /* possibly AND it with a user requested mask */
>> smp_call_function_many(mask, ipi_func, ....);
>> 
>> which I think will be faster on some archs (that support broadcast)
>> and have fewer problems with out of sync values (though we do have to
>> check in our IPI function that we haven't context switched out.
>> 
>> Am I missing why this won't work?
> 
> My impression is that some architectures don't provide the needed
> ordering in this case, and also that some architectures support ASIDs
> and would thus IPI CPUs that weren't actually running threads in the
> process at the current time.
> 
> Mathieu, anything I am missing?

As per my other email, it's pretty much it, yes.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com