Re: Udpated sys_membarrier() speedup patch, FYI

Mathieu Desnoyers
2017-07-27T21:00:22Z

----- On Jul 27, 2017, at 4:58 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Jul 27, 2017, at 4:37 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com
> wrote:
>
>> On Thu, Jul 27, 2017 at 11:04:13PM +0300, Avi Kivity wrote:
[...]
>>> >+
>>> >+ this_cpu = raw_smp_processor_id();
>>> >+ for_each_online_cpu(cpu) {
>>> >+ struct task_struct *p;
>>> >+
>>> >+ if (cpu == this_cpu)
>>> >+ continue;
>>> >+ rcu_read_lock();
>>> >+ p = task_rcu_dereference(&cpu_rq(cpu)->curr);
>>> >+ if (p && p->mm == current->mm)
>>> >+ __cpumask_set_cpu(cpu, tmpmask);
>>>
>>> This gets you some false positives, if the CPU idled then mm will
>>> not have changed.
>>
>> Good point! The battery-powered embedded guys would probably prefer
>> we not needlessly IPI idle CPUs. We cannot rely on RCU's dyntick-idle
>> state in nohz_full cases. Not sure if is_idle_task() can be used
>> safely, given things like play_idle().
>
> Would changing the check in this loop to:
>
> if (p && !is_idle_task(p) && p->mm == current->mm) {
>
> work for you ?

Avi, is there an optimization that allows current->mm to be non-null
when the idle task is scheduled that I am missing ?

I would have expected current->mm to be always NULL for idle
tasks.

Thanks,

Mathieu

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

--
Mathieu Desnoyers
EfficiOS Inc.