Re: [RFC][PATCH v3 6/6] rcu/tree: Use irq_work_queue_remote()

[Peter Zijlstra]

On Wed, Oct 28, 2020 at 03:54:28PM +0100, Peter Zijlstra wrote:
> On Wed, Oct 28, 2020 at 12:07:13PM +0100, Peter Zijlstra wrote:
> > AFAICT we only need/use irq_work_queue_on() on remote CPUs, since we
> > can directly access local state.  So avoid the IRQ_WORK dependency and
> > use the unconditionally available irq_work_queue_remote().
> > 
> > This survives a number of TREE01 runs.
> 
> OK, Paul mentioned on IRC that while it is extremely unlikely, this code
> does not indeed guarantee it will not try to IPI self.
> 
> I'll try again.

This is the best I could come up with.. :/

---
Subject: rcu/tree: Use irq_work_queue_remote()
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Wed Oct 28 11:53:40 CET 2020

All sites that consume rcu_iw_gp_seq seem to have rcu_node lock held,
so setting it probably should too. Also the effect of self-IPI here
would be setting rcu_iw_gp_seq to the value we just set it to
(pointless) and clearing rcu_iw_pending, which we just set, so don't
set it.

Passes TREE01.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/rcu/tree.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1308,14 +1308,16 @@ static int rcu_implicit_dynticks_qs(stru
 			resched_cpu(rdp->cpu);
 			WRITE_ONCE(rdp->last_fqs_resched, jiffies);
 		}
-#ifdef CONFIG_IRQ_WORK
+		raw_spin_lock_rcu_node(rnp);
 		if (!rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
 		    (rnp->ffmask & rdp->grpmask)) {
-			rdp->rcu_iw_pending = true;
 			rdp->rcu_iw_gp_seq = rnp->gp_seq;
-			irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
+			if (likely(rdp->cpu != smp_processor_id())) {
+				rdp->rcu_iw_pending = true;
+				irq_work_queue_remote(rdp->cpu, &rdp->rcu_iw);
+			}
 		}
-#endif
+		raw_spin_unlock_rcu_node(rnp);
 	}
 
 	return 0;