Date:	Tue, 22 Mar 2016 14:19:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Chatre, Reinette" <reinette.chatre@...el.com>
Cc:	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Josh Triplett <josh@...htriplett.org>,
	Ross Green <rgkernel@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	"dipankar@...ibm.com" <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	pranith kumar <bobby.prani@...il.com>
Subject: Re: rcu_preempt self-detected stall on CPU from 4.5-rc3, since 3.17

On Tue, Mar 22, 2016 at 09:04:47PM +0000, Chatre, Reinette wrote:
> Hi Paul,
> 
> On 2016-03-22, Paul E. McKenney wrote:
> > On Tue, Mar 22, 2016 at 04:35:32PM +0000, Chatre, Reinette wrote:
> >> On 2016-03-21, Paul E. McKenney wrote:
> >>> On Mon, Mar 21, 2016 at 09:22:30AM -0700, Jacob Pan wrote:
> >>>> On Fri, 18 Mar 2016 16:56:41 -0700
> >>>> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> >>>>> 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:
> >>> 
> >>> [ . . . ]
> >>> 
> >>>>>> 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...
> > 
> > And thank you for the .config.  Your kenrle version looks to be 4.5.0.
> > 
> >>>> +Reinette, she has the system that can reproduce the issue. I
> >>>> believe she is having some other problems with it at the moment. But
> >>>> the .config should be available. Version is v4.5.
> >>> 
> >>> A couple of additional questions:
> >>> 
> >>> 1.	Is the test running on bare metal or virtualized?  If the
> >>> 	latter, what is the host?
> >> 
> >> Bare metal.
> > 
> > OK, you are ahead of me.  Mine is virtualized.
> > 
> >>> 2.	Does the workload involve CPU hotplug?
> >> 
> >> No.
> > 
> > Again, you are ahead of me.  Mine makes extremely heavy use of CPU hotplug.
> > 
> >>> 3.	Are you seeing things like this in dmesg?
> >>> 
> >>> 	"rcu_preempt kthread starved for 21033 jiffies"
> >>> 	"rcu_sched kthread starved for 32103 jiffies"
> >>> 	"rcu_bh kthread starved for 84031 jiffies"
> >>> 
> >>> 	If not, you are probably facing some other bug, and should
> >>> 	proceed debugging as described in Documentation/RCU/stallwarn.txt.
> >> 
> >> Below is a sample of what I see as captured with v4.5. The kernel
> >> configuration is attached.
> >> 
> >> [  135.456197] INFO: rcu_preempt detected stalls on CPUs/tasks: [ 
> >> 135.457729]  3-...: (0 ticks this GP) idle=722/0/0 softirq=5532/5532
> >> fqs=0 [  135.459604]  (detected by 2, t=60004 jiffies, g=2105, c=2104,
> >> q=165) [  135.461318] Task dump for CPU 3: [  135.461321] swapper/3    
> >>   R  running task        0     0      1 0x00200000 [  135.461325] 
> >> 00000078560040e5 ffff88017846fed0 ffffffff818af2cc ffff880100000000 [ 
> >> 135.461330]  0000000600000003 ffff880178470000 ffff880072f32200
> >> ffffffff822dcec0 [  135.461334]  ffff88017846c000 ffff88017846c000
> >> ffff88017846fee0 ffffffff818af517 [  135.461338] Call Trace: [ 
> >> 135.461345]  [<ffffffff818af2cc>] ? cpuidle_enter_state+0xfc/0x310 [ 
> >> 135.461349]  [<ffffffff818af517>] ? cpuidle_enter+0x17/0x20 [ 
> >> 135.461353]  [<ffffffff811515aa>] ? call_cpuidle+0x2a/0x40 [ 
> >> 135.461355]  [<ffffffff8115197d>] ? cpu_startup_entry+0x28d/0x360 [ 
> >> 135.461360]  [<ffffffff8108c874>] ? start_secondary+0x114/0x140 [ 
> >> 135.461365] rcu_preempt kthread starved for 60004 jiffies! g2105 c2104
> > f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1
> > 
> > And yes, it looks like you are seeing the same bug that I am tracing.
> > 
> > The kthread is blocked on a schedule_timeout_interruptible().  Given
> > default configuration, this would have a three-jiffy timeout.
> > 
> > You set CONFIG_RCU_CPU_STALL_TIMEOUT=60, which matches the 60004 jiffies
> > above.  Is that value due to a distro setting or something?  Mainline
> > uses CONFIG_RCU_CPU_STALL_TIMEOUT=21.
> 
> Indeed ... this value originated from a Fedora configuration. 

OK.  Setting it shorter might (or might not) make it reproduce more
quickly.  This can be set at boot time via rcupdate.rcu_cpu_stall_timeout.
Or at compile time via CONFIG_RCU_CPU_STALL_TIMEOUT.

> >> [  135.463965] rcu_preempt     S ffff88017844fd68     0     7      2
> >> 0x00000000 [  135.463969]  ffff88017844fd68 ffff88017dd8cc80
> >> ffff880177ff0000 ffff880178443b80 [  135.463973]  ffff880178450000
> >> ffff88017844fda0 ffff88017dd8cc80 ffff88017dd8cc80 [  135.463977] 
> >> 0000000000000003 ffff88017844fd80 ffffffff81ab031f 0000000100031504 [ 
> >> 135.463981] Call Trace: [  135.463986]  [<ffffffff81ab031f>]
> >> schedule+0x3f/0xa0 [  135.463989]  [<ffffffff81ab42d7>]
> >> schedule_timeout+0x127/0x270 [  135.463993]  [<ffffffff81171a50>] ?
> >> detach_if_pending+0x120/0x120 [  135.463997]  [<ffffffff8116da5d>]
> >> rcu_gp_kthread+0x6bd/0xa30 [  135.464000]  [<ffffffff81151390>] ?
> >> wake_atomic_t_function+0x70/0x70 [  135.464003]  [<ffffffff8116d3a0>] ?
> >> force_qs_rnp+0x1b0/0x1b0 [  135.464006]  [<ffffffff8112f846>]
> >> kthread+0xe6/0x100 [  135.464009]  [<ffffffff8112f760>] ?
> >> kthread_worker_fn+0x190/0x190 [  135.464012]  [<ffffffff81ab5c0f>]
> >> ret_from_fork+0x3f/0x70 [  135.464015]  [<ffffffff8112f760>] ?
> >> kthread_worker_fn+0x190/0x190
> > 
> > How long does it take to reproduce this?  If it reproduces in minutes
> > or hours, could you please boot with the following on the kernel command
> > line and dump the trace buffer shortly after the stall?
> > 
> > ftrace trace_event=sched_waking,sched_wakeup,sched_wake_idle_without_ipi
> 
> The trace I provided above appeared after a few minutes and not again. On previous occasions I had to wait a few hours. I tried running with the above added to the kernel command line but I have not seen the trace yet. I will leave the system overnight but then may risk not capturing the data you need so ...

Fair enough...  Sounds like you might have the same geologic-time
problem that I do when adding tracing.  :-/

> > If dumping manually shortly after the stall is at all non-trivial
> > (for example, if your reproduction time is many minute or hours),
> > I can supply some patches that automate this.  Or you can pick
> > them up from -rcu:
> 
> ... could you please point me to the patches you refer to? Or would you like me to try with the entire kernel from rcu/dev?

2dc92e2a86b9 (rcu: Awaken grace-period kthread if too long since FQS)
c3fd2095d015 (rcu: Dump ftrace buffer when kicking grace-period kthread)

There might be other dependencies, but these are the two that you need.

							Thanx, Paul

> > git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
> > 
> > Branch rcu/dev has these patches (and much else besides).
> > 
> > 							Thanx, Paul
> > 
> > PS:  In case you are curious, when I enable those tracepoints, it
> >      shows me that the timer is firing every three jiffies, as it
> >      should, but that something happens between the sched_waking
> >      and the IPI handler that should actually do the wakeup.
> >      However, adding the traces significantly slows reproduction,
> >      so I am writing a stress test specific to this bug to try to
> >      speed things up, hopefully allowing more tracing to be added
> >      while still retaining non-geologic reproduction times.
> 
> Thank you very much for these details. 
> 
> Reinette
> 

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