Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs

[Paul E. McKenney]

On Tue, Feb 15, 2022 at 07:53:10PM +0530, Mukesh Ojha wrote:
> 
> On 2/14/2022 10:14 PM, Paul E. McKenney wrote:
> > On Thu, Feb 10, 2022 at 12:38:11AM +0100, Frederic Weisbecker wrote:
> > > On Fri, Feb 04, 2022 at 02:55:07PM -0800, Paul E. McKenney wrote:
> > > > Although it is usually safe to invoke synchronize_rcu_expedited() from a
> > > > preemption-enabled CPU-hotplug notifier, if it is invoked from a notifier
> > > > between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its attempts to
> > > > invoke a workqueue handler will hang due to RCU waiting on a CPU that
> > > > the scheduler is not paying attention to.  This commit therefore expands
> > > > use of the existing workqueue-independent synchronize_rcu_expedited()
> > > > from early boot to also include CPUs that are being hotplugged.
> > > > 
> > > > Link:https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@xxxxxxxxxxx/
> > > > Reported-by: Mukesh Ojha<quic_mojha@xxxxxxxxxxx>
> > > > Cc: Tejun Heo<tj@xxxxxxxxxx>
> > > > Signed-off-by: Paul E. McKenney<paulmck@xxxxxxxxxx>
> > > I'm surprised by this scheduler behaviour.
> > > 
> > > Since sched_cpu_activate() hasn't been called yet,
> > > rq->balance_callback = balance_push_callback. As a result, balance_push() should
> > > be called at the end of schedule() when the workqueue is picked as the next task.
> > > Then eventually the workqueue should be immediately preempted by the stop task to
> > > be migrated elsewhere.
> > > 
> > > So I must be missing something. For the fun, I booted the following and it
> > > didn't produce any issue:
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 80faf2273ce9..b1e74a508881 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -4234,6 +4234,8 @@ int rcutree_online_cpu(unsigned int cpu)
> > >   	// Stop-machine done, so allow nohz_full to disable tick.
> > >   	tick_dep_clear(TICK_DEP_BIT_RCU);
> > > +	if (cpu != 0)
> > > +		synchronize_rcu_expedited();
> > >   	return 0;
> > >   }
> > That does seem compelling.  And others have argued that the workqueue
> > system's handling of offline CPUs should deal with this.
> > 
> > Mukesh, was this a theoretical bug, or did you actually make it happen?
> > If you made it happen, as seems to have been the case given your original
> > email [1], could you please post your reproducer?
> 
> No, it was not theoretical one. We saw this issue only once in our testing
> and i don't think it is easy to reproduce otherwise
> it would been fixed by now.
> 
> When one of thread calling synchronize_expedite_rcu with timer of 20s but it
> did not get the exp funnel
> lock for 20s and there we crash it with panic() on timeout.
> 
> The other thread cpuhp which was having the lock got stuck at the point
> mentioned at the below link.
> 
> https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@xxxxxxxxxxx/

OK.  Are you able to create an in-kernel reproducer, perhaps similar to
Frederic's change above?

I am worried that the patch that I am carrying might be fixing some
other bug by accident...

							Thanx, Paul

> e.g Below sample test in combination of many other test in parallel
> 
> :loop
> 
> adb shell "echo 0 > /sys/devices/system/cpu/cpu0/online"
> 
> adb shell "echo 1 > /sys/devices/system/cpu/cpu0/online"
> 
> adb shell "echo 0 > /sys/devices/system/cpu/cpu1/online"
> 
> adb shell "echo 1 > /sys/devices/system/cpu/cpu1/online"
> 
> adb shell "echo 0 > /sys/devices/system/cpu/cpu2/online"
> 
> adb shell "echo 1 > /sys/devices/system/cpu/cpu2/online"
> 
> adb shell "echo 0 > /sys/devices/system/cpu/cpu3/online"
> 
> adb shell "echo 1 > /sys/devices/system/cpu/cpu3/online"
> 
> adb shell "echo 0 > /sys/devices/system/cpu/cpu4/online"
> 
> adb shell "echo 1 > /sys/devices/system/cpu/cpu4/online"
> 
> adb shell "echo 0 > /sys/devices/system/cpu/cpu5/online"
> 
> adb shell "echo 1 > /sys/devices/system/cpu/cpu5/online"
> 
> adb shell "echo 0 > /sys/devices/system/cpu/cpu6/online"
> 
> adb shell "echo 1 > /sys/devices/system/cpu/cpu6/online"
> 
> adb shell "echo 0 > /sys/devices/system/cpu/cpu7/online"
> 
> adb shell "echo 1 > /sys/devices/system/cpu/cpu7/online"
> 
> goto loop
> 
> 
> 
> Thanks, Mukesh
> 
> > 
> > 							Thanx, Paul
> > 
> > [1]https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@xxxxxxxxxxx/