Re: [PATCH RFC tip/core/rcu 2/5] rcu: Short-circuit normal GPs via expedited GPs

[Paul E. McKenney]

On Wed, Jul 01, 2015 at 12:03:52PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 30, 2015 at 02:48:27PM -0700, Paul E. McKenney wrote:
> > @@ -2121,17 +2137,24 @@ static int __noreturn rcu_gp_kthread(void *arg)
> >  					       TPS("fqswait"));
> >  			rsp->gp_state = RCU_GP_WAIT_FQS;
> >  			ret = wait_event_interruptible_timeout(rsp->gp_wq,
> > -					((gf = READ_ONCE(rsp->gp_flags)) &
> > -					 RCU_GP_FLAG_FQS) ||
> > -					(!READ_ONCE(rnp->qsmask) &&
> > -					 !rcu_preempt_blocked_readers_cgp(rnp)),
> > -					j);
> > +				((gf = READ_ONCE(rsp->gp_flags)) &
> > +				 RCU_GP_FLAG_FQS) ||
> > +				(!READ_ONCE(rnp->qsmask) &&
> > +				 !rcu_preempt_blocked_readers_cgp(rnp)) ||
> > +				rcu_exp_gp_seq_done(rsp->exp_rsp,
> > +						    rsp->gp_exp_snap),
> > +				j);
> 
> How about using a helper function there?
> 
> static inline bool rcu_gp_done(rsp, rnp)
> {
> 	/* Forced Quiescent State complete */
> 	if (READ_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS)
> 		return true;
> 
> 	/* QS not masked and not blocked by preempted readers */
> 	if (!READ_ONCE(rnp->qsmask) &&
> 	    !rcu_preempt_blocked_readers_cgp(rnp))
> 		return true;
> 
> 	/* Expedited Grace Period completed */
> 	if (rcu_exp_gp_seq_done(rsp))
> 		return true;
> 
> 	return false;
> }
> 
> 	ret = wait_event_interruptible_timeout(rsp->gp_wq,
> 		rcu_gp_done(rsp, rnp), j);

Fair point, and applies to the original as well, give or take comments
and naming.  Please see below for the corresponding patch.  Thoughts?

							Thanx, Paul

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

commit a8bb9abbf3690e507d61efe8144cec091ce5fb02
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date:   Wed Jul 1 13:50:28 2015 -0700

    rcu: Pull out wait_event*() condition into helper function
    
    The condition for the wait_event_interruptible_timeout() that waits
    to do the next force-quiescent-state scan is a bit ornate:
    
    	((gf = READ_ONCE(rsp->gp_flags)) &
    	 RCU_GP_FLAG_FQS) ||
    	(!READ_ONCE(rnp->qsmask) &&
    	 !rcu_preempt_blocked_readers_cgp(rnp))
    
    This commit therefore pulls this condition out into a helper function
    and comments its component conditions.
    
    Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4992bfd360b6..2afb8e8c5134 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1904,6 +1904,26 @@ static int rcu_gp_init(struct rcu_state *rsp)
 }
 
 /*
+ * Helper function for wait_event_interruptible_timeout() wakeup
+ * at force-quiescent-state time.
+ */
+static bool rcu_gp_fqs_check_wake(struct rcu_state *rsp, int *gfp)
+{
+	struct rcu_node *rnp = rcu_get_root(rsp);
+
+	/* Someone like call_rcu() requested a force-quiescent-state scan. */
+	*gfp = READ_ONCE(rsp->gp_flags);
+	if (*gfp & RCU_GP_FLAG_FQS)
+		return true;
+
+	/* The current grace period has completed. */
+	if (!READ_ONCE(rnp->qsmask) && !rcu_preempt_blocked_readers_cgp(rnp))
+		return true;
+
+	return false;
+}
+
+/*
  * Do one round of quiescent-state forcing.
  */
 static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
@@ -2067,11 +2087,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
 					       TPS("fqswait"));
 			rsp->gp_state = RCU_GP_WAIT_FQS;
 			ret = wait_event_interruptible_timeout(rsp->gp_wq,
-					((gf = READ_ONCE(rsp->gp_flags)) &
-					 RCU_GP_FLAG_FQS) ||
-					(!READ_ONCE(rnp->qsmask) &&
-					 !rcu_preempt_blocked_readers_cgp(rnp)),
-					j);
+					rcu_gp_fqs_check_wake(rsp, &gf), j);
 			rsp->gp_state = RCU_GP_DONE_FQS;
 			/* Locking provides needed memory barriers. */
 			/* If grace period done, leave loop. */

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