Re: [PATCH v2 -rcu dev 2/3] rcu/tree: Fix issue where sometimes rcu_urgent_qs is not set on IPI
From: Paul E. McKenney
Date: Fri Aug 16 2019 - 13:05:05 EST
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);
}