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

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

On Thu, Mar 21, 2019 at 04:32:44PM -0700, Paul E. McKenney wrote:
> On Wed, Mar 20, 2019 at 04:46:01PM -0700, Paul E. McKenney 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.
> > > 
> > > Reported-by: Thomas Gleixner <tglx@linutronix.de>
> > > Tested-by: Mike Galbraith <efault@gmx.de>
> > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > 
> > Thank you!  I reverted v2 and applied this one with the same sort of
> > update.  Testing is going well thus far aside from my failing to add
> > the required "=0" after the rcutree.use_softirq.  I will probably not
> > be the only one who will run afoul of this, so I updated the commit log
> > and the documentation accordingly, as shown below.
> 
> And I took a look, please see updates/questions interspersed.
> 
> I didn't find anything substantive, but still I get hangs.  Which is
> the normal situation.  ;-)
> 
> Will fire off more testing...

And despite my protestations about restrictions involving the scheduler
and rcu_read_unlock(), with the patch below TREE01, TREE02, TREE03, and
TREE09 pass an hour of rcutorture with rcutree.use_softirq=0.  Without
this patch, seven-minute runs get hard hangs and this:

[   18.417315] BUG: spinlock recursion on CPU#5, rcu_torture_rea/763
[   18.418624]  lock: 0xffff9d207eb61940, .magic: dead4ead, .owner: rcu_torture_rea/763, .owner_cpu: 5
[   18.420418] CPU: 5 PID: 763 Comm: rcu_torture_rea Not tainted 5.1.0-rc1+ #1
[   18.421786] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[   18.423375] Call Trace:
[   18.423880]  <IRQ>
[   18.424284]  dump_stack+0x46/0x5b
[   18.424953]  do_raw_spin_lock+0x8d/0x90
[   18.425699]  try_to_wake_up+0x2cd/0x4f0
[   18.426493]  invoke_rcu_core_kthread+0x63/0x80
[   18.427337]  rcu_read_unlock_special+0x41/0x80
[   18.428212]  __rcu_read_unlock+0x48/0x50
[   18.428984]  cpuacct_charge+0x96/0xd0
[   18.429725]  ? cpuacct_charge+0x2e/0xd0
[   18.430463]  update_curr+0x112/0x240
[   18.431172]  enqueue_task_fair+0xa9/0x1220
[   18.432009]  ttwu_do_activate+0x49/0xa0
[   18.432741]  sched_ttwu_pending+0x75/0xa0
[   18.433583]  scheduler_ipi+0x53/0x150
[   18.434291]  reschedule_interrupt+0xf/0x20
[   18.435137]  </IRQ

I clearly need to audit the setting of ->rcu_read_unlock_special.

Again, the patch below is bad for expedited grace periods, so it is
experimental.

							Thanx, Paul

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

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ca972b0b2467..d133fa837426 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -607,12 +607,9 @@ static void rcu_read_unlock_special(struct task_struct *t)
 	if (preempt_bh_were_disabled || irqs_were_disabled) {
 		WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
 		/* Need to defer quiescent state until everything is enabled. */
-		if (irqs_were_disabled) {
+		if (irqs_were_disabled && use_softirq) {
 			/* Enabling irqs does not reschedule, so... */
-			if (use_softirq)
-				raise_softirq_irqoff(RCU_SOFTIRQ);
-			else
-				invoke_rcu_core();
+			raise_softirq_irqoff(RCU_SOFTIRQ);
 		} else {
 			/* Enabling BH or preempt does reschedule, so... */
 			set_tsk_need_resched(current);