Re: Udpated sys_membarrier() speedup patch, FYI

From	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Newsgroups	linux.kernel
Subject	Re: Udpated sys_membarrier() speedup patch, FYI
Date	2017-07-27 23:10 +0200
Message-ID	<u7YcF-2T1-5@gated-at.bofh.it> (permalink)
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----- 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.
http://www.efficios.com

Thread

Udpated sys_membarrier() speedup patch, FYI "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 20:20 +0200
  Re: Udpated sys_membarrier() speedup patch, FYI Andrew Hunter <ahh@google.com> - 2017-07-27 20:40 +0200
    Re: Udpated sys_membarrier() speedup patch, FYI "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 21:10 +0200
      Re: Udpated sys_membarrier() speedup patch, FYI Andrew Hunter <ahh@google.com> - 2017-07-28 19:40 +0200
        Re: Udpated sys_membarrier() speedup patch, FYI "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-28 20:20 +0200
  Re: Udpated sys_membarrier() speedup patch, FYI Avi Kivity <avi@scylladb.com> - 2017-07-27 21:30 +0200
    Re: Udpated sys_membarrier() speedup patch, FYI "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 21:50 +0200
      Re: Udpated sys_membarrier() speedup patch, FYI Avi Kivity <avi@scylladb.com> - 2017-07-27 22:10 +0200
        Re: Udpated sys_membarrier() speedup patch, FYI "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-27 22:40 +0200
          Re: Udpated sys_membarrier() speedup patch, FYI Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-27 23:00 +0200
            Re: Udpated sys_membarrier() speedup patch, FYI Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-27 23:10 +0200
              Re: Udpated sys_membarrier() speedup patch, FYI Avi Kivity <avi@scylladb.com> - 2017-07-31 08:10 +0200
                Re: Udpated sys_membarrier() speedup patch, FYI Peter Zijlstra <peterz@infradead.org> - 2017-07-31 10:40 +0200
                Re: Udpated sys_membarrier() speedup patch, FYI Avi Kivity <avi@scylladb.com> - 2017-07-31 11:00 +0200
      Re: Udpated sys_membarrier() speedup patch, FYI Andrew Hunter <ahh@google.com> - 2017-07-28 19:20 +0200
        Re: Udpated sys_membarrier() speedup patch, FYI Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-28 19:30 +0200
        Re: Udpated sys_membarrier() speedup patch, FYI "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-28 19:40 +0200
          Re: Udpated sys_membarrier() speedup patch, FYI Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-07-28 19:50 +0200

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