Re: [PATCH 1/2] rcu: Assign higher priority to RCU threads if its rcutorture

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Jun 18, 2018 at 11:22:14PM -0700, Joel Fernandes wrote:
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> 
> rcutorture boost tests fail even with CONFIG_RCU_BOOST set because
> rcutorture's threads are equal priority to the default RCU kthreads (RT
> class with priority of 1).

Sorry for the weird subject line, I meant "rcu: Assign higher prio if
rcutorture is built into kernel". I have included the patch with the subject
line fixed up below (if you prefer to take that instead).

Also one question, incase rcutorture is a module, we can't raise the priority
of the kthreads because it would be too late to do at module load time. In
this case, do you have any ideas on what we can do? I was thinking we can
access the kernel command line from within rcutorture module and check if
'rcutree.kthread_prio' was passed. And if it is and isn't sufficiently high,
then we avoid testing boost feature at all (and print a nice message telling
the user about the issue).

OTOH, we can just let rcutorture module loaders fail the test if you feel
very few automation tests do the module loading way of rcutorture and so a
boost test failure there is tolerable. For me, I will likely be running
rcutorture only as a built-in so I am Ok with not special casing it within
rcutorture.

thanks!

 - Joel

-----8<---------

From 8cb7c2ac98e510abac35fdf2419a3212a587095a Mon Sep 17 00:00:00 2001
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Date: Mon, 18 Jun 2018 15:13:10 -0700
Subject: [PATCH 1/2] rcu: Assign higher prio if rcutorture is built into kernel

rcutorture boost tests fail even with CONFIG_RCU_BOOST set because
rcutorture's threads are equal priority to the default RCU kthreads (RT
class with priority of 1).

This patch checks if RCU torture is built into the kernel and if so,
assigns a higher priority to the RCU threads. With this the rcutorture
boost tests pass.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/rcu/tree.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index deb2508be923..a141d6314622 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -171,7 +171,7 @@ static void rcu_report_exp_rdp(struct rcu_state *rsp,
 static void sync_sched_exp_online_cleanup(int cpu);
 
 /* rcuc/rcub kthread realtime priority */
-static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
+static int kthread_prio;
 module_param(kthread_prio, int, 0644);
 
 /* Delay in jiffies for grace-period initialization delays, debug only. */
@@ -3884,12 +3884,16 @@ static int __init rcu_spawn_gp_kthread(void)
 	struct task_struct *t;
 
 	/* Force priority into range. */
-	if (IS_ENABLED(CONFIG_RCU_BOOST) && kthread_prio < 1)
+	if (IS_ENABLED(CONFIG_RCU_BOOST) && kthread_prio < 2
+	    && IS_BUILTIN(CONFIG_RCU_TORTURE_TEST))
+		kthread_prio = 2;
+	else if (IS_ENABLED(CONFIG_RCU_BOOST) && kthread_prio < 1)
 		kthread_prio = 1;
 	else if (kthread_prio < 0)
 		kthread_prio = 0;
 	else if (kthread_prio > 99)
 		kthread_prio = 99;
+
 	if (kthread_prio != kthread_prio_in)
 		pr_alert("rcu_spawn_gp_kthread(): Limited prio to %d from %d\n",
 			 kthread_prio, kthread_prio_in);
-- 
2.18.0.rc1.244.gcf134e6275-goog