Messages in this thread | | | Date | Mon, 18 Jun 2018 18:08:03 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [lkp-robot] [rcutorture] 46e26223e3: WARNING:at_kernel/rcu/rcutorture.c:#rcu_torture_stats_print |
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On Mon, Jun 18, 2018 at 03:26:47PM -0700, Joel Fernandes wrote: > On Mon, Jun 18, 2018 at 09:56:46AM -0700, Paul E. McKenney wrote: > > > The reason for the rcutorture test failure could be that the default > > > kthread_prio for the system's RCU threads is set to 1 (unless overridden by > > > rcutree.kthread_prio) which is also equal to the priority of the rcutorture's > > > boost threads. Due to this the rcutorture test could starve the RCU threads > > > as well and defeat the boosting mechanism. I was able to solve a similar > > > issue by just passing rcutree.kthread_prio of 50 on the kernel command line. > > > > > > Paul, would it be ok if we changed the default kthread_prio to something > 1 > > > so that rcutorture can test properly without needing to pass any extra > > > rcutree.* parameters? > > > > > > so something like this in kernel/rcu/tree.c ? > > > > > > static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 2 : 0; > > > > Would it be possible to also condition this on rcutorture being built > > in? Or are they doing modprobes for rcutorture? > > They seem to be doing built-in rcutorture tests. But I believe the same > problem would occur even if you used modules? I believe the fact that > rcutorture is a module or built-in wouldn't matter to the underlying issue > which is the RCU subsystems's threads are at too low of a priority > (rcutree.kthread_prio = 1).
Understood...
> If you agree with changing the default priority, I have included a patch > below for rcu/dev.
The problem is that without rcutorture, rcutree.kthread_prio=1 is a legitimate choice, and changing the default globally could be breaking someone. So it would be far better to up the priority only during known rcutorture testing.
Thanx, Paul
> thanks, > > - Joel > > ---8<----------------------- > > >From b0f4111ef1abd1c481c269fadb3535c83ab43c93 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] rcu: Change default RCU kthread priority to 2 > > The current RT priority of 1 for RCU kthreads makes rcutorture's boost test > fail on systems where rcutree.kthread_prio isn't passed. > > The rcutorture boost kthreads have the same priority as well (RT priority of > 1). Due to this, the rcutorture kthreads starve the RCU subsystem's kthreads > and causes rcutorture failures. This patch changes the priority of the RCU > subsystem's threads to a default RT priority of 2 so that rcutorture's > threads get preempted by them. Verified that the boost tests will pass with > this change. > > Reported-by: Xiaolong Ye (via lkp-robot) <xiaolong.ye@intel.com> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > --- > kernel/rcu/tree.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index deb2508be923..920c39e3f871 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 = IS_ENABLED(CONFIG_RCU_BOOST) ? 2 : 0; > module_param(kthread_prio, int, 0644); > > /* Delay in jiffies for grace-period initialization delays, debug only. */ > -- > 2.18.0.rc1.244.gcf134e6275-goog >
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