On Thu, Aug 15, 2019 at 10:59:14PM -0400, Joel Fernandes (Google) wrote: > Sometimes I see rcu_urgent_qs is not set. This could be when the last > IPI was a long time ago, however, the grace period just started. Set > rcu_urgent_qs so the tick can indeed not be stopped. > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Great catch, queued, thank you! I updated the commit log as shown below, so could you please check to see if I messed anything up? Thanx, Paul ------------------------------------------------------------------------ commit 1f80df1e49b29ccd605882056a0565778e100e9e Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Date: Thu Aug 15 22:59:14 2019 -0400 rcu/tree: Ensure that ->rcu_urgent_qs is set before resched IPI The RCU-specific resched_cpu() function sends a resched IPI to the specified CPU, which can be used to force the tick on for a given nohz_full CPU. This is needed when this nohz_full CPU is looping in the kernel while blocking the current grace period. However, for the tick to actually be forced on in all cases, that CPU's rcu_data structure's ->rcu_urgent_qs flag must be set beforehand. This commit therefore causes rcu_implicit_dynticks_qs() to set this flag prior to invoking resched_cpu() on a holdout nohz_full CPU. Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 0512de9ead20..dd0a77bffa7d 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1091,6 +1091,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp) if (tick_nohz_full_cpu(rdp->cpu) && time_after(jiffies, READ_ONCE(rdp->last_fqs_resched) + jtsq * 3)) { + WRITE_ONCE(*ruqp, true); resched_cpu(rdp->cpu); WRITE_ONCE(rdp->last_fqs_resched, jiffies); }