Re: Udpated sys_membarrier() speedup patch, FYI

[Andrew Hunter <ahh@google.com>]

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.

> +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?