Re: Udpated sys_membarrier() speedup patch, FYI

Avi Kivity
2017-07-31T06:03:15Z

On 07/28/2017 12:02 AM, Mathieu Desnoyers wrote:
> ----- 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.
>
>

I remembered that current->mm does not change when switching to a kernel
task, but my Kernlish is very rusty, or maybe it has changed.