Re: Udpated sys_membarrier() speedup patch, FYI

[Avi Kivity]

On 07/28/2017 12:02 AM, Mathieu Desnoyers wrote:
> ----- On Jul 27, 2017, at 4:58 PM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx wrote:
>
> > ----- On Jul 27, 2017, at 4:37 PM, Paul E. McKenney paulmck@xxxxxxxxxxxxxxxxxx
> > 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.