Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop() enable tick on all online CPUs

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

On Sun, Aug 04, 2019 at 04:48:35PM +0200, Peter Zijlstra wrote:
> On Sun, Aug 04, 2019 at 04:43:17PM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 02, 2019 at 08:15:01AM -0700, Paul E. McKenney wrote:
> > > The multi_cpu_stop() function relies on the scheduler to gain control from
> > > whatever is running on the various online CPUs, including any nohz_full
> > > CPUs running long loops in kernel-mode code.  Lack of the scheduler-clock
> > > interrupt on such CPUs can delay multi_cpu_stop() for several minutes
> > > and can also result in RCU CPU stall warnings.  This commit therefore
> > > causes multi_cpu_stop() to enable the scheduler-clock interrupt on all
> > > online CPUs.
> > 
> > This sounds wrong; should we be fixing sched_can_stop_tick() instead to
> > return false when the stop task is runnable?

Agreed.  However, it is proving surprisingly hard to come up with a
code sequence that has the effect of rcu_nocb without nohz_full.
And rcu_nocb works just fine.  With nohz_full also in place, I am
decreasing the failure rate, but it still fails, perhaps a few times
per hour of TREE04 rcutorture on an eight-CPU system.  (My 12-CPU
system stubbornly refuses to fail.  Good thing I kept the eight-CPU
system around, I guess.)

When I arrive at some sequence of actions that actually work reliably,
then by all means let's put it somewhere in the NO_HZ_FULL machinery!

> And even without that; I don't understand how we're not instantly
> preempted the moment we enqueue the stop task.

There is no preemption because CONFIG_PREEMPT=n for the scenarios still
having trouble.  Yes, there are cond_resched() calls, but they don't do
anything unless the appropriate flags are set, which won't always happen
without the tick, apparently.  Or without -something- that isn't always
happening as it should.

> Any enqueue, should go through check_preempt_curr() which will be an
> instant resched_curr() when we just woke the stop class.

I did try hitting all of the CPUs with resched_cpu().  Ten times on each
CPU with a ten-jiffy wait between each.  This might have decreased the
probability of excessively long CPU-stopper waits by a factor of two or
three, but it did not eliminate the excessively long waits.

What else should I try?

For example, are there any diagnostics I could collect, say from within
the CPU stopper when things are taking too long?  I see CPU-stopper
delays in excess of five -minutes-, so this is anything but subtle.

							Thanx, Paul