Re: suspicious RCU due to "Prefer using an idle CPU as a migration target instead of comparing tasks"

[Qian Cai]

On Thu, 2020-02-27 at 15:26 +0000, Valentin Schneider wrote:
> On Thu, Feb 27 2020, Qian Cai wrote:
> 
> > On Thu, 2020-02-27 at 09:09 -0500, Qian Cai wrote:
> > > The linux-next commit ff7db0bf24db ("sched/numa: Prefer using an idle CPU as a
> > > migration target instead of comparing tasks") introduced a boot warning,
> > 
> > This?
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index a61d83ea2930..ca780cd1eae2 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1607,7 +1607,9 @@ static void update_numa_stats(struct task_numa_env *env,
> > if (ns->idle_cpu == -1)
> > ns->idle_cpu = cpu;
> > 
> > +rcu_read_lock();
> > idle_core = numa_idle_core(idle_core, cpu);
> > +rcu_read_unlock();
> > }
> > }
> > 
> 
> 
> Hmph right, we have
> numa_idle_core()->test_idle_cores()->rcu_dereference().
> 
> Dunno if it's preferable to wrap the entirety of update_numa_stats() or
> if that fine-grained read-side section is ok.

I could not come up with a better fine-grained one than this.

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a61d83ea2930..980d03fa157c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1562,9 +1562,16 @@ static inline int numa_idle_core(int idle_core, int cpu)
Â{
Â#ifdef CONFIG_SCHED_SMT
Â	if (!static_branch_likely(&sched_smt_present) ||
-	ÂÂÂÂidle_core >= 0 || !test_idle_cores(cpu, false))
-		return idle_core;
+	ÂÂÂÂidle_core >= 0) {
+		bool idle;
Â
+		rcu_read_lock();
+		idle = test_idle_cores(cpu, false);
+		rcu_read_unlock();
+
+		if (!idle)
+			return idle_core;
+	}
Â	/*
Â	Â* Prefer cores instead of packing HT siblings
Â	Â* and triggering future load balancing.