Re: rcu_preempt self-detected stall on CPU from 4.5-rc3, since 3.17

[Paul E. McKenney]

On Fri, Mar 18, 2016 at 02:00:11PM -0700, Josh Triplett wrote:
> On Thu, Feb 25, 2016 at 04:56:38PM -0800, Paul E. McKenney wrote:
> > On Thu, Feb 25, 2016 at 04:13:11PM +1100, Ross Green wrote:
> > > On Wed, Feb 24, 2016 at 8:28 AM, Ross Green <rgkernel@xxxxxxxxx> wrote:
> > > > On Wed, Feb 24, 2016 at 7:55 AM, Paul E. McKenney
> > > > <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > 
> > [ . . . ]
> > 
> > > >> Still working on getting decent traces...
> > 
> > And I might have succeeded, see below.
> > 
> > > >>
> > > >>                                                         Thanx, Paul
> > > >>
> > > >
> > > > G'day all,
> > > >
> > > > Here is another dmesg output for 4.5-rc5 showing another rcu_preempt stall.
> > > > This one appeared after only a day of running. CONFIG_DEBUG_TIMING is
> > > > turned on, but can't see any output that shows from this.
> > > >
> > > > Again testing as before,
> > > >
> > > > Boot, run a series of small benchmarks, then just let the system be
> > > > and idle away.
> > > >
> > > > I notice in the stack trace there is mention of hrtimer_run_queues and
> > > > hrtimer_interrupt.
> > > >
> > > > Anyway, leave this for a few more eyes to look at.
> > > >
> > > > Open to any other suggestions of things to test.
> > > >
> > > > Regards,
> > > >
> > > > Ross Green
> > > 
> > > 
> > > G'day Paul,
> > > 
> > > I left the pandaboard running and captured another stall.
> > > 
> > > the attachment is the dmesg output.
> > > 
> > > Again there is no apparent output from any CONFIG_DEBUG_TIMING so I
> > > assume there is nothing happening there.
> > 
> > I agree, looks like this is not due to time skew.
> > 
> > > I just saw the updates for 4.6 RCU code.
> > > Is the patch in [PATCH tip/core/rcu 04/13] valid here?
> > 
> > I doubt that it will help, but you never know.
> > 
> > > do you want me try the new patch set with this configuration?
> > 
> > Even better would be to try Daniel Wagner's swait patchset.  I have
> > attached them in UNIX mbox format, or you can get them from the
> > -tip tree.
> > 
> > And I -finally- got some tracing that -might- be useful.  The dmesg, all
> > 67MB of it, is here:
> > 
> > 	http://www.rdrop.com/~paulmck/submission/console.2016.02.23a.log
> > 
> > This failure mode is less likely to happen, and looks a bit different
> > than the ones that I was seeing before enabling tracing.  Then, an
> > additional wakeup would actually wake the task up.  In contrast, with
> > tracing enabled, the RCU grace-period kthread goes into "teenager mode",
> > refusing to wake up despite repeated attempts.  However, this might
> > be a side-effect of the ftrace dump.
> > 
> > On line 525,132, we see that the rcu_preempt grace-period kthread has
> > been starved for 1,188,154 jiffies, or about 20 minutes.  This seems
> > unlikely...  The kthread is waiting for no more than a three-jiffy
> > timeout ("RCU_GP_WAIT_FQS(3)") and is in TASK_INTERRUPTIBLE state
> > ("0x1").
> 
> We're seeing a similar stall (~60 seconds) on an x86 development system
> here.  Any luck tracking down the cause of this?  If not, any
> suggestions for traces that might be helpful?

The dmesg containing the stall, the kernel version, and the .config would
be helpful!  Working on a torture test specific to this bug...

							Thanx, Paul