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SubjectRe: [PATCH] rcu: Make jiffies_till_sched_qs writable
On Thu, Jul 11, 2019 at 03:58:39PM -0400, Joel Fernandes wrote:
> Hmm, speaking of grace period durations, it seems to me the maximum grace
> period ever is recorded in rcu_state.gp_max. However it is not read from
> anywhere.
> Any idea why it was added but not used?
> I am interested in dumping this value just for fun, and seeing what I get.
> I wonder also it is useful to dump it in rcutorture/rcuperf to find any
> issues, or even expose it in sys/proc fs to see what worst case grace periods
> look like.


commit ae91aa0adb14dc33114d566feca2f7cb7a96b8b7
rcu: Remove debugfs tracing

removed all debugfs tracing, gp_max also included.

And you sounds great. And even looks not that hard to add it like,


diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ad9dc86..86095ff 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1658,8 +1658,10 @@ static void rcu_gp_cleanup(void)
rcu_state.gp_end = jiffies;
gp_duration = rcu_state.gp_end - rcu_state.gp_start;
- if (gp_duration > rcu_state.gp_max)
+ if (gp_duration > rcu_state.gp_max) {
rcu_state.gp_max = gp_duration;
+ trace_rcu_grace_period(something something);
+ }

* We know the grace period is complete, but to everyone else

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