linux-kernel - Re: Udpated sys_membarrier() speedup patch, FYI

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Date:   Thu, 27 Jul 2017 21:02:24 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Avi Kivity <avi@...lladb.com>
Cc:     maged michael <maged.michael@...il.com>,
        Andrew Hunter <ahh@...gle.com>, gromer <gromer@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: Udpated sys_membarrier() speedup patch, FYI

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

> ----- On Jul 27, 2017, at 4:37 PM, Paul E. McKenney paulmck@...ux.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.
http://www.efficios.com