Date:   Fri, 13 Apr 2018 22:38:13 +0200
From:   Niklas Söderlund 
        <niklas.soderlund@...natech.se>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-renesas-soc@...r.kernel.org,
        Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: Potential problem with 31e77c93e432dec7 ("sched/fair: Update
 blocked load when newly idle")

Hi Vincent,

On 2018-04-12 13:15:19 +0200, Niklas Söderlund wrote:
> Hi Vincent,
> 
> Thanks for your feedback.
> 
> On 2018-04-12 12:33:27 +0200, Vincent Guittot wrote:
> > Hi Niklas,
> > 
> > On 12 April 2018 at 11:18, Niklas Söderlund
> > <niklas.soderlund@...natech.se> wrote:
> > > Hi Vincent,
> > >
> > > I have observed issues running on linus/master from a few days back [1].
> > > I'm running on a Renesas Koelsch board (arm32) and I can trigger a issue
> > > by X forwarding the v4l2 test application qv4l2 over ssh and moving the
> > > courser around in the GUI (best test case description award...). I'm
> > > sorry about the really bad way I trigger this but I can't do it in any
> > > other way, I'm happy to try other methods if you got some ideas. The
> > > symptom of the issue is a complete hang of the system for more then 30
> > > seconds and then this information is printed in the console:
> > 
> > Heiner (edded cc) also reported similar problem with his platform: a
> > dual core celeron
> > 
> > Do you confirm that your platform is a dual cortex-A15 ? At least that
> > what I have seen on web
> > This would confirm that dual system is a key point.
> 
> I can confirm that my platform is a dual core.

I tested another dual core system today Renesas M3-W ARM64 system and I 
can observe the same lockups-on that system if it helps you understand 
the problem. It seems to be much harder to trigger the issue on this 
system for some reason. Hitting return in a ssh session don't seem to 
produce the lockup while starting a GUI using X forwarding over ssh it's 
possible.

[  392.306441] INFO: rcu_preempt detected stalls on CPUs/tasks:
[  392.312201]  (detected by 0, t=19366 jiffies, g=7177, c=7176, q=35)
[  392.318555] All QSes seen, last rcu_preempt kthread activity 19368 
(4294990375-4294971007), jiffies_till_next_fqs=1, root ->qsmask 0x0
[  392.330758] swapper/0       R  running task        0     0      0 
0x00000022
[  392.337883] Call trace:
[  392.340365]  dump_backtrace+0x0/0x1c8
[  392.344065]  show_stack+0x14/0x20
[  392.347416]  sched_show_task+0x224/0x2e8
[  392.351377]  rcu_check_callbacks+0x8ac/0x8b0
[  392.355686]  update_process_times+0x2c/0x58
[  392.359908]  tick_sched_handle.isra.5+0x30/0x50
[  392.364479]  tick_sched_timer+0x40/0x90
[  392.368351]  __hrtimer_run_queues+0xfc/0x208
[  392.372659]  hrtimer_interrupt+0xd4/0x258
[  392.376710]  arch_timer_handler_virt+0x28/0x48
[  392.381194]  handle_percpu_devid_irq+0x80/0x138
[  392.385767]  generic_handle_irq+0x28/0x40
[  392.389813]  __handle_domain_irq+0x5c/0xb8
[  392.393946]  gic_handle_irq+0x58/0xa8
[  392.397640]  el1_irq+0xb4/0x130
[  392.400810]  arch_cpu_idle+0x14/0x20
[  392.404422]  default_idle_call+0x1c/0x38
[  392.408381]  do_idle+0x17c/0x1f8
[  392.411640]  cpu_startup_entry+0x20/0x28
[  392.415598]  rest_init+0x24c/0x260
[  392.419037]  start_kernel+0x3e8/0x414

I was running the same tests on another ARM64 platform earlier using the 
same build which have more then two cores and there I could not observe 
this issue.

-- 
Regards,
Niklas Söderlund

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