Re: rcu_preempt self-detected stall on CPU from 4.5-rc3, since 3.17
From: Paul E. McKenney
Date: Tue Mar 22 2016 - 17:19:45 EST
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@xxxxxxxxxxxxxxxxxx> 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.
> > 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.