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From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: linux-kernel@vger.kernel.org, andrea.parri@amarulasolutions.com
Subject: Re: Question about sched_setaffinity()
Date: Tue, 30 Apr 2019 12:03:18 +0200	[thread overview]
Message-ID: <20190430100318.GP2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190427180246.GA15502@linux.ibm.com>

On Sat, Apr 27, 2019 at 11:02:46AM -0700, Paul E. McKenney wrote:

> This actually passes rcutorture.  But, as Andrea noted, not klitmus.
> After some investigation, it turned out that klitmus was creating kthreads
> with PF_NO_SETAFFINITY, hence the failures.  But that prompted me to
> put checks into my code: After all, rcutorture can be fooled.
> 
> 	void synchronize_rcu(void)
> 	{
> 		int cpu;
> 
> 		for_each_online_cpu(cpu) {
> 			sched_setaffinity(current->pid, cpumask_of(cpu));
> 			WARN_ON_ONCE(raw_smp_processor_id() != cpu);
> 		}
> 	}
> 
> This triggers fairly quickly, usually in less than a minute of rcutorture
> testing.
>
> And further investigation shows that sched_setaffinity()
> always returned 0. 

> Is this expected behavior?  Is there some configuration or setup that I
> might be missing?

ISTR there is hotplug involved in RCU torture? In that case, it can be
sched_setaffinity() succeeds to place us on a CPU, which CPU hotplug
then takes away. So when we run the WARN thingy, we'll be running on a
different CPU than expected.

If OTOH, your loop is written like (as it really should be):

	void synchronize_rcu(void)
	{
		int cpu;

		cpus_read_lock();
		for_each_online_cpu(cpu) {
			sched_setaffinity(current->pid, cpumask_of(cpu));
			WARN_ON_ONCE(raw_smp_processor_id() != cpu);
		}
		cpus_read_unlock();
	}

Then I'm not entirely sure how we can return 0 and not run on the
expected CPU. If we look at __set_cpus_allowed_ptr(), the only paths out
to 0 are:

 - if the mask didn't change
 - if we already run inside the new mask
 - if we migrated ourself with the stop-task
 - if we're not in fact running

That last case should never trigger in your circumstances, since @p ==
current and current is obviously running. But for completeness, the
wakeup of @p would do the task placement in that case.


  reply	other threads:[~2019-04-30 10:03 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-27 18:02 Paul E. McKenney
2019-04-30 10:03 ` Peter Zijlstra [this message]
2019-04-30 10:51   ` Paul E. McKenney
2019-04-30 11:55     ` Peter Zijlstra
2019-05-01 19:12       ` Paul E. McKenney
2019-05-01 19:16         ` Steven Rostedt
2019-05-01 20:27           ` Paul E. McKenney
2019-05-07 22:16             ` Paul E. McKenney
2019-05-09 17:36               ` Paul E. McKenney
2019-05-09 19:36                 ` Paul E. McKenney
2019-05-10 12:08                   ` Peter Zijlstra
2019-05-10 23:07                     ` Paul E. McKenney
2019-05-11 21:45                       ` Andrea Parri
2019-05-12  0:39                         ` Paul E. McKenney
2019-05-12  1:05                           ` Andrea Parri
2019-05-13 12:20                             ` Paul E. McKenney
2019-05-13 15:37                               ` Joel Fernandes
2019-05-13 15:53                                 ` Paul E. McKenney
2019-05-13  8:10                       ` Peter Zijlstra
2019-05-13 12:19                         ` Paul E. McKenney
2019-05-09 21:40                 ` Andrea Parri
2019-05-09 21:56                   ` Andrea Parri
2019-05-09 22:17                     ` Paul E. McKenney
2019-05-10  6:32                       ` Andrea Parri

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