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/
