Re: sched/core warning triggers on rcu torture test

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

On Tue, Jun 26, 2018 at 06:32:55PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 26, 2018 at 06:16:04PM +0200, Anna-Maria Gleixner wrote:
> > Hi,
> > 
> > during rcu torture tests (TREE04 and TREE07) I noticed, that a
> > WARN_ON_ONCE() in sched core triggers on a recent 4.18-rc2 based
> > kernel (6f0d349d922b ("Merge
> > git://git.kernel.org/pub/scm/linux/kernel/git/davem/net")) as well as
> > on a 4.17.3.

First, I am very glad that I am not the only one running rcutorture!  ;-)

> > I'm running the tests on a machine with 144 cores:
> > 
> >   tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 144 --duration 120 --configs "9*TREE07"
> >   tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 144 --duration 120 --configs "18*TREE04"
> > 
> > 
> > The warning was introduced by commit d84b31313ef8 ("sched/isolation:
> > Offload residual 1Hz scheduler tick").
> > 
> > 
> > Output looks similar for all tests I did (this one is the output of
> > the 4.18-rc2 based kernel):
> > 
> > WARNING: CPU: 11 PID: 906 at kernel/sched/core.c:3138 sched_tick_remote+0xb6/0xc0
> 
> That's nohz_full stuff, is that a normal part of rcutorture? In any
> case, is the one housekeeping CPU getting seriously overloaded or
> something?

Yes, nohz_full is a normal part for rcutorture because RCU has to deal
differently with userspace execution in the nohz_full case.

I do see this splat (at least when I don't comment it out), but I
do share my system with others, so I could easily be overloading the
housekeeping vCPUs due to hypervisor preemption.  I was intending to
dig into this one once I got done consolidating RCU-bh, RCU-preempt,
and RCU-sched at Linus's behest.

On overloading the housekeeping CPU without outside load, let's look at
TREE04 and TREE07 separately.

TREE04 uses eight CPUs, and seven of them ("nohz_full=1-7") are nohz_full
CPUs, and rcutorture doesn't generate all that large of a callback load.
It looks like all 144 CPUs are used in this case (18*8), though RCU
enforces idle periods in order to test idle/non-idle transitions.
But was there anything else running on the machine at the time?

TREE07 uses 16 CPUs, and eight of them ("nohz_full=2-9") are nohz_full
CPUs.  Again, it looks like all 144 CPUs are used (9*8).

I sometimes see this on TASKS03 as well, which uses two CPUs, and one of
them ("nohz_full=1") is a nohz_full CPU.

If your system is otherwise idle, would it make sense to trace context
switches on CPU 0 to see what it is up to?  And to do an ftrace_dump()
and turn tracing off when the warning triggers as well?

							Thanx, Paul