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

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

On Sun, Mar 27, 2016 at 10:45:59PM +0200, Peter Zijlstra wrote:
> On Sun, Mar 27, 2016 at 08:40:18AM -0700, Paul E. McKenney wrote:
> > Oh, and the patch I am running with is below.  I am running x86, and so
> > some other architectures would of course need the corresponding patch
> > on that architecture.
> 
> > -#define TIF_POLLING_NRFLAG	21	/* idle is polling for TIF_NEED_RESCHED */
> > +/* #define TIF_POLLING_NRFLAG	21	 idle is polling for TIF_NEED_RESCHED */
> 
> x86 is the only arch that really uses this heavily IIRC.
> 
> Most of the other archs need interrupts to wake up remote cores.
> 
> So what we try to do is avoid sending IPIs when the CPU is idle, for the
> remote wakeup case we use set_nr_if_polling() which sets
> TIF_NEED_RESCHED if TIF_POLLING_NRFLAG was set. If it wasn't, we'll send
> the IPI. Otherwise we rely on the idle loop to do sched_ttwu_pending()
> when it breaks out of loop due to TIF_NEED_RESCHED.
> 
> But, you need hotplug for this to happen, right?

I do, but Ross Green is seeing something that looks similar, and without
CPU hotplug.

> We should not be migrating towards, or waking on, CPUs no longer present
> in cpu_active_map, and there is a rcu/sched_sync() after clearing that
> bit. Furthermore, migration_call() does a sched_ttwu_pending() (waking
> any remaining stragglers) before we migrate all runnable tasks off the
> dying CPU.

OK, so I should instrument migration_call() if I get the repro rate up?

> The other interesting case would be resched_cpu(), which uses
> set_nr_and_not_polling() to kick a remote cpu to call schedule(). It
> atomically sets TIF_NEED_RESCHED and returns if TIF_POLLING_NRFLAG was
> not set. If indeed not, it will send an IPI.
> 
> This assumes the idle 'exit' path will do the same as the IPI does; and
> if you look at cpu_idle_loop() it does indeed do both
> preempt_fold_need_resched() and sched_ttwu_pending().
> 
> Note that one cannot rely on irq_enter()/irq_exit() being called for the
> scheduler IPI.

OK, thank you for the info!  Any specific debug actions?

							Thanx, Paul