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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Mar 28, 2016 at 03:07:36PM +0000, Mathieu Desnoyers wrote:
> ----- On Mar 28, 2016, at 9:29 AM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:
> 
> > On Mon, Mar 28, 2016 at 08:28:51AM +0200, Peter Zijlstra wrote:
> >> On Sun, Mar 27, 2016 at 02:09:14PM -0700, Paul E. McKenney wrote:
> >> 
> >> > > Does that system have MONITOR/MWAIT errata?
> >> > 
> >> > On the off-chance that this question was also directed at me,
> >> 
> >> Hehe, it wasn't, however, since we're here..
> >> 
> >> > here is
> >> > what I am running on.  I am running in a qemu/KVM virtual machine, in
> >> > case that matters.
> >> 
> >> Have you actually tried on real proper hardware? Does it still reproduce
> >> there?
> > 
> > Ross has, but I have not, given that I have a shared system on the one
> > hand and a single-socket (four core, eight hardware thread) laptop on
> > the other that has even longer reproduction times.  The repeat-by is
> > as follows:
> > 
> > o	Build a kernel with the following Kconfigs:
> > 
> >	CONFIG_SMP=y
> >	CONFIG_NR_CPUS=16
> >	CONFIG_PREEMPT_NONE=n
> >	CONFIG_PREEMPT_VOLUNTARY=n
> >	CONFIG_PREEMPT=y
> >	# This should result in CONFIG_PREEMPT_RCU=y
> >	CONFIG_HZ_PERIODIC=y
> >	CONFIG_NO_HZ_IDLE=n
> >	CONFIG_NO_HZ_FULL=n
> >	CONFIG_RCU_TRACE=y
> >	CONFIG_HOTPLUG_CPU=y
> >	CONFIG_RCU_FANOUT=2
> >	CONFIG_RCU_FANOUT_LEAF=2
> >	CONFIG_RCU_NOCB_CPU=n
> >	CONFIG_DEBUG_LOCK_ALLOC=n
> >	CONFIG_RCU_BOOST=y
> >	CONFIG_RCU_KTHREAD_PRIO=2
> >	CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
> >	CONFIG_RCU_EXPERT=y
> >	CONFIG_RCU_TORTURE_TEST=y
> >	CONFIG_PRINTK_TIME=y
> >	CONFIG_RCU_TORTURE_TEST_SLOW_CLEANUP=y
> >	CONFIG_RCU_TORTURE_TEST_SLOW_INIT=y
> >	CONFIG_RCU_TORTURE_TEST_SLOW_PREINIT=y
> > 
> >	If desired, you can instead build with CONFIG_RCU_TORTURE_TEST=m
> >	and modprobe/insmod the module manually.
> > 
> > o	Find a two-socket x86 system or larger, with at least 16 CPUs.
> > 
> > o	Boot the kernel with the following kernel boot parameters:
> > 
> >	rcutorture.onoff_interval=1 rcutorture.onoff_holdoff=30
> > 
> >	The onoff_holdoff is only needed for CONFIG_RCU_TORTURE_TEST=y.
> >	When manually setting up the module, you get the holdoff for
> >	free, courtesy of human timescales.
> > 
> > In the absence of instrumentation, I get failures usually within a
> > couple of hours, though sometimes much longer.  With instrumentation,
> > the sky appears to be the limit.  :-/
> > 
> > Ross is running on bare metal with no CPU hotplug, so perhaps his setup
> > is of more immediate interest.  He is seeing the same symptoms that I am,
> > namely a task being repeatedly awakened without actually coming out of
> > TASK_INTERRUPTIBLE state, let alone running.  As you pointed out earlier,
> > he cannot be seeing the same bug that my crude patch suppresses, but
> > given that I still see a few failures with that crude patch, it is quite
> > possible that there is still a common bug.
> 
> With respect to bare metal vs KVM guest, I've reported an issue with
> inaccurate detection of TSC as being an unreliable time source on a
> KVM guest. The basic setup is to overcommit the CPU use across the
> entire host, thus leading to preemption of the guest. The guest TSC
> watchdog then falsely assume that TSC is unreliable, because it gets
> preempted for a long time (e.g. 0.5 second) between reading the HPET
> and the TSC.
> 
> Ref. http://lkml.iu.edu/hypermail/linux/kernel/1509.1/00379.html
> 
> I'm wondering if what Paul is observing in the KVM setup might be
> caused by long preemption by the host. One way to stress test this
> is to run parallel kernel builds on the host (or in another guest)
> while the guest is running, thus over-committing the CPU use.
> 
> Thoughts ?

If I run NO_HZ_FULL, I do get warnings about unstable timesources.

And certainly guest VCPUs can be preempted.  However, if they were
preempted for the lengths of time I am seeing, I should also see
softlockup warnings on the host, which I do not see.

That said, perhaps I should cobble together something to force short
repeated preemptions at the host level.  Maybe that would get the
reproduction rate sufficiently high to enable less-dainty debugging.

							Thanx, Paul