Re: [PATCH v3] rcu: Allow to eliminate softirq processing from rcutree

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Mon, Mar 25, 2019 at 08:08:00AM -0700, Paul E. McKenney wrote:
> On Mon, Mar 25, 2019 at 09:41:29AM -0400, Joel Fernandes wrote:
> > On Sun, Mar 24, 2019 at 04:42:11PM -0700, Paul E. McKenney wrote:
> > > On Sat, Mar 23, 2019 at 09:10:02AM -0700, Paul E. McKenney wrote:
> > > > On Fri, Mar 22, 2019 at 05:25:19PM -0700, Paul E. McKenney wrote:
> > > > > On Fri, Mar 22, 2019 at 07:48:19PM -0400, Joel Fernandes wrote:
> > > > > > On Wed, Mar 20, 2019 at 10:13:33PM +0100, Sebastian Andrzej Siewior wrote:
> > > > > > > Running RCU out of softirq is a problem for some workloads that would
> > > > > > > like to manage RCU core processing independently of other softirq
> > > > > > > work, for example, setting kthread priority.  This commit therefore
> > > > > > > introduces the `rcunosoftirq' option which moves the RCU core work
> > > > > > > from softirq to a per-CPU/per-flavor SCHED_OTHER kthread named rcuc.
> > > > > > > The SCHED_OTHER approach avoids the scalability problems that appeared
> > > > > > > with the earlier attempt to move RCU core processing to from softirq
> > > > > > > to kthreads.  That said, kernels built with RCU_BOOST=y will run the
> > > > > > > rcuc kthreads at the RCU-boosting priority.
> > > > > > [snip]
> > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > > > index 0f31b79eb6761..05a1e42fdaf10 100644
> > > > > > > --- a/kernel/rcu/tree.c
> > > > > > > +++ b/kernel/rcu/tree.c
> > > > > > > @@ -51,6 +51,12 @@
> > > > > > >  #include <linux/tick.h>
> > > > > > >  #include <linux/sysrq.h>
> > > > > > >  #include <linux/kprobes.h>
> > > > > > > +#include <linux/gfp.h>
> > > > > > > +#include <linux/oom.h>
> > > > > > > +#include <linux/smpboot.h>
> > > > > > > +#include <linux/jiffies.h>
> > > > > > > +#include <linux/sched/isolation.h>
> > > > > > > +#include "../time/tick-internal.h"
> > > > > > >  
> > > > > > >  #include "tree.h"
> > > > > > >  #include "rcu.h"
> > > > > > > @@ -92,6 +98,9 @@ struct rcu_state rcu_state = {
> > > > > > >  /* Dump rcu_node combining tree at boot to verify correct setup. */
> > > > > > >  static bool dump_tree;
> > > > > > >  module_param(dump_tree, bool, 0444);
> > > > > > > +/* Move RCU_SOFTIRQ to rcuc kthreads. */
> > > > > > > +static bool use_softirq = 1;
> > > > > > > +module_param(use_softirq, bool, 0444);
> > > > > > >  /* Control rcu_node-tree auto-balancing at boot time. */
> > > > > > >  static bool rcu_fanout_exact;
> > > > > > >  module_param(rcu_fanout_exact, bool, 0444);
> > > > > > > @@ -2253,7 +2262,7 @@ void rcu_force_quiescent_state(void)
> > > > > > >  EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);
> > > > > > >  
> > > > > > >  /* Perform RCU core processing work for the current CPU.  */
> > > > > > > -static __latent_entropy void rcu_core(struct softirq_action *unused)
> > > > > > > +static __latent_entropy void rcu_core(void)
> > > > > > >  {
> > > > > > >  	unsigned long flags;
> > > > > > >  	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
> > > > > > > @@ -2295,6 +2304,34 @@ static __latent_entropy void rcu_core(struct softirq_action *unused)
> > > > > > >  	trace_rcu_utilization(TPS("End RCU core"));
> > > > > > >  }
> > > > > > >  
> > > > > > > +static void rcu_core_si(struct softirq_action *h)
> > > > > > > +{
> > > > > > > +	rcu_core();
> > > > > > > +}
> > > > > > > +
> > > > > > > +static void rcu_wake_cond(struct task_struct *t, int status)
> > > > > > > +{
> > > > > > > +	/*
> > > > > > > +	 * If the thread is yielding, only wake it when this
> > > > > > > +	 * is invoked from idle
> > > > > > > +	 */
> > > > > > > +	if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current)))
> > > > > > > +		wake_up_process(t);
> > > > > > > +}
> > > > > > > +
> > > > > > > +static void invoke_rcu_core_kthread(void)
> > > > > > > +{
> > > > > > > +	struct task_struct *t;
> > > > > > > +	unsigned long flags;
> > > > > > > +
> > > > > > > +	local_irq_save(flags);
> > > > > > > +	__this_cpu_write(rcu_data.rcu_cpu_has_work, 1);
> > > > > > > +	t = __this_cpu_read(rcu_data.rcu_cpu_kthread_task);
> > > > > > > +	if (t != NULL && t != current)
> > > > > > > +		rcu_wake_cond(t, __this_cpu_read(rcu_data.rcu_cpu_kthread_status));
> > > > > > > +	local_irq_restore(flags);
> > > > > > > +}
> > > > > > > +
> > > > > > >  /*
> > > > > > >   * Schedule RCU callback invocation.  If the running implementation of RCU
> > > > > > >   * does not support RCU priority boosting, just do a direct call, otherwise
> > > > > > > @@ -2306,19 +2343,95 @@ static void invoke_rcu_callbacks(struct rcu_data *rdp)
> > > > > > >  {
> > > > > > >  	if (unlikely(!READ_ONCE(rcu_scheduler_fully_active)))
> > > > > > >  		return;
> > > > > > > -	if (likely(!rcu_state.boost)) {
> > > > > > > -		rcu_do_batch(rdp);
> > > > > > > -		return;
> > > > > > > -	}
> > > > > > > -	invoke_rcu_callbacks_kthread();
> > > > > > > +	if (rcu_state.boost || !use_softirq)
> > > > > > > +		invoke_rcu_core_kthread();
> > > > > > > +	rcu_do_batch(rdp);
> > > > > > 
> > > > > > Shouldn't there be an else before the rcu_do_batch? If we are waking up the
> > > > > > rcuc thread, then that will do the rcu_do_batch when it runs right?
> > > > > > 
> > > > > > Something like:
> > > > > > 	if (rcu_state.boost || !use_softirq)
> > > > > > 		invoke_rcu_core_kthread();
> > > > > > 	else
> > > > > > 		rcu_do_batch(rdp);
> > > > > > 
> > > > > > Previous code similarly had a return; also.
> > > > > 
> > > > > I believe that you are correct, so I will give it a shot.  Good eyes!
> > > > 
> > > > Yet rcutorture disagrees.  Actually, if we are using rcuc kthreads, this
> > > > is only ever invoked from within tha thread, so the only check we need is
> > > > for the scheduler being operational.  I am therefore trying this one out.
> > > > 
> > > > Thoughts?
> > > 
> > > And rcutorture likes this one, though at this point this function should
> > > be pulled into its sole callsite.  ;-)
> > 
> > Great, I'm glad the testing is going well.
> 
> Which reminds me...  I have been assuming that Frederic Weisbecker's
> split-softirq patches were stalled for the time being.
> 
> http://lkml.kernel.org/r/20190228171242.32144-1-frederic@kernel.org
> 
> If those were to show up soonish, perhaps that would allow per-softirq
> control of priority.
> 
> My thought is not to wait, but I figured I should mention it.
> 
> > By the way I enlightened that jitter.sh script about CPU offline issues as
> > well (sent patch last week).  Let me know if you agree with it.
> 
> I just sent a reply.  Still trying to remember why I excluded CPU 0.  ;-)
> 
> Perhaps because of issues with single-CPU rcutorture runs?

I also considered and rejected the following patch because it actually
can make sense to build with CONFIG_RCU_BOOST but still use softirq, for
example, when SCHED_IDLE tasks might get stuck in RCU read-side critical
sections.  But then I noticed that rcu_spawn_core_kthreads() unconditionally
creates the rcuc kthreads if CONFIG_RCU_BOOST.

So I either need to apply the patch below, or I need to remove
the "!IS_ENABLED(CONFIG_RCU_BOOST)" from the "if" statement in
rcu_spawn_core_kthreads().  The question is "do we allow CONFIG_RCU_BOOST
kernels to use RCU_SOFTIRQ?"  Some plusses and minuses:

+	Supports the SCHED_IDLE use case for CONFIG_RCU_BOOST without
	slowing down other workloads.  This might be important given
	RCU flavor consolidation

-	Another configuration combination to test and maintain.

So I am leaning towards ditching the patch below in favor of updating
the "if" condition in rcu_spawn_core_kthreads().

Thoughts?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a17034ee4d3d..5782fe9ac27d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -99,8 +99,12 @@ struct rcu_state rcu_state = {
 static bool dump_tree;
 module_param(dump_tree, bool, 0444);
 /* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */
+#ifdef CONFIG_RCU_BOOST
+static const bool use_softirq = 0;
+#else /* #ifdef CONFIG_RCU_BOOST */
 static bool use_softirq = 1;
 module_param(use_softirq, bool, 0444);
+#endif /* #else #ifdef CONFIG_RCU_BOOST */
 /* Control rcu_node-tree auto-balancing at boot time. */
 static bool rcu_fanout_exact;
 module_param(rcu_fanout_exact, bool, 0444);