linux-kernel - Re: [PATCH] rcu: Make jiffies_till_sched

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Date:   Fri, 12 Jul 2019 15:32:40 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>, josh@...htriplett.org,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [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.

Hi,

	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)
 	raw_spin_lock_irq_rcu_node(rnp);
 	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

Thanks,
Byungchul