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

[Paul E. McKenney]

On Sun, Aug 04, 2019 at 11:41:59AM -0700, Paul E. McKenney wrote:
> 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.

For whatever it is worth, the things on my list include using 25 rounds
of resched_cpu() on each CPU with ten-jiffy wait between each (instead of
merely 10 rounds), using waitqueues or some such to actually force a
meaningful context switch on the other CPUs, etc.

							Thanx, Paul