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

[Paul E. McKenney]

On Mon, Aug 05, 2019 at 10:05:31AM +0200, Peter Zijlstra wrote:
> 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!
> 
> I'm confused; what are you arguing? The patch as proposed is just wrong,
> it needs to go.

Eventually, sure.  But one dragon at a time.  Right now that dragon is
"what is required to get multi_cpu_stop() to work in a timely fashioon".
The "where does that code really go" dragon comes later.

> > > 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
> 
> That doesn't make sense; even with CONFIG_PREEMPT=n we set
> TIF_NEED_RESCHED. We'll just not react to it as promptly (only explicit
> rescheduling points and return to userspace). Enabling the tick will not
> make any difference what so ever.
> 
> Tick based preemption will not 'fix' the lack of wakeup preemption. If
> the stop task wakeup didn't set TIF_NEED_RESCHED, the OTHER/CFS tick
> will not either.

Seems logical except for the fact that multi_cpu_stop() really is taking
in excess of five minutes on a regular basis.

> > 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.
> 
> Right; so clearly we're not understanding what's happening. That seems
> like a requirement for actually doing a patch.

Almost but not quite.  It is a requirement for a patch *that* *is*
*supposed* *to* *be* *a* *fix*.  If you are trying to prohibit me from
writing experimental patches, please feel free to take a long walk on
a short pier.

Understood???

> > > 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.
> 
> Catch the whole thing in a function trace?
> 
> The chain that should instantly set TIF_NEED_RESCHED:
> 
>   stop_machine()
>     stop_machine_cpuslocked()
>       stop_cpus()
>         __stop_cpus()
>           queue_stop_cpus_work()
>             cpu_stop_queue_work()
> 	      wake_up_q()
> 	        wake_up_process()
> 
> 
>   wake_up_process()
>     try_to_wake_up()
>       ttwu_queue()
>         ttwu_queue_remote()
> 	  <- scheduler_ipi()
> 	    sched_ttwu_pending()
> 	      ttwu_do_activate()
> 
>         ttwu_do_activate()
> 	  activate_task()
> 	  ttwu_do_wakeup()
> 	    check_preempt_curr()
> 	      resched_curr()
> 
> You could frob some tracing into __stop_cpus(), before
> wait_for_completion(), at that point all the CPUs in @cpumask should
> either be running the stop task or have TIF_NEED_RESCHED set.

Thank you, this should be quite helpful.

							Thanx, Paul