On Wed, Oct 12, 2016 at 11:12:51AM -0400, Luiz Capitulino wrote:

Hi,

We have the following patch applied to the RT tree:

  commit a9d3cc781a3306bfa332fa7cb6134b70696058d5
  Author: Josh Cartwright [off-list ref]
  Date:   Tue Oct 27 07:31:53 2015 -0500
  
      net: Make synchronize_rcu_expedited() conditional on !RT_FULL

However, as noted by Michael, making rcu_normal=1 default in the
RT kernel should have the same effect (ie.  not calling
synchronize_sched_expedited()).

So, honest question, is there a reason not to:

 1. Drop the patch above
 2. Make rcu_normal=1 default?

I think this is probably a cleaner way to fix the problems which
motivated this patch in the first place.  In my defense, the above patch
predates rcu_normal :).

Something like this, perhaps?

   Julia

-- 8< --
Subject: [PATCH] rcu: enable rcu_normal_after_boot by default for RT

The forcing of an expedited grace period is an expensive and very
RT-application unfriendly operation, as it forcibly preempts all running
tasks on CPUs which are preventing the gp from expiring.

By default, as a policy decision, disable the expediting of grace
periods (after boot) on configurations which enable PREEMPT_RT_FULL.

Suggested-by: Luiz Capitulino <redacted>
Signed-off-by: Julia Cartwright <redacted>
---
 kernel/rcu/update.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index b40d346..4975917 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c

@@ -63,7 +63,7 @@ MODULE_ALIAS("rcupdate");
 #ifndef CONFIG_TINY_RCU
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
-static int rcu_normal_after_boot;
+static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
 module_param(rcu_normal_after_boot, int, 0);
 #endif /* #ifndef CONFIG_TINY_RCU */
 

-- 
2.9.3