Re: [PATCH 0/2] rcu: Fix series of spurious RCU softirqs

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 11/24/2010 08:31 AM, Frederic Weisbecker wrote:
> Hi,
> 
> I've observed some not so unfrequent series of spurious rcu
> softirqs, sometimes happening at each ticks for a random
> while.
> 
> These patches aims at fixing them.
> 
> Thanks.
> 
> Frederic Weisbecker (2):
>   rcu: Don't chase unnecessary quiescent states after extended grace periods
>   rcu: Stop checking quiescent states after grace period completion from remote
> 

If we ensure rdp->gpnum >= rdp->completed is always true, the problems as
you described will not be existed. Or maybe I misunderstand you.

rdp->gpnum >= rdp->completed is a very important guarantee I think.
(In my RCURING, it is guaranteed.) I'm afraid there are some other
problems still hidden if it is not guaranteed.

so I recommend: (code is better than words)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d5bc439..af4e87a 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -648,6 +648,13 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
 
 		/* Remember that we saw this grace-period completion. */
 		rdp->completed = rnp->completed;
+
+		/* Ensure ->gpnum >= ->completed after NO_HZ */
+		if (unlikely(rnp->completed - rdp->gpnum > 0
+				|| rdp->gpnum - rnp->gpnum > 0)) {
+			rdp->gpnum = rnp->completed;
+			rdp->qs_pending = 0;
+		}
 	}
 }