On Wed, Aug 28, 2019 at 01:13:44PM -0700, Paul E. McKenney wrote:

On Mon, Aug 26, 2019 at 09:33:53PM -0400, Joel Fernandes (Google) wrote:

quoted

The dynticks counter are confusing due to crowbar writes of
DYNTICK_IRQ_NONIDLE whose purpose is to detect half-interrupts (i.e. we
see rcu_irq_enter() but not rcu_irq_exit() due to a usermode upcall) and
if so then do a reset of the dyntick_nmi_nesting counters. This patch
tries to get rid of DYNTICK_IRQ_NONIDLE while still keeping the code
working, fully functional, and less confusing. The confusion recently
has even led to patches forgetting that DYNTICK_IRQ_NONIDLE was written
to which wasted lots of time.

The patch has the following changes:

[snip]

quoted

 /*
  * Grace-period counter management.
  */

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 68ebf0eb64c8..255cd6835526 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c

@@ -81,7 +81,7 @@
 
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
 	.dynticks_nesting = 1,
-	.dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
+	.dynticks_nmi_nesting = 0,

C initializes to zero by default, so this can simply be deleted.

Fixed.

quoted

 	.dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR),
 };
 struct rcu_state rcu_state = {

@@ -558,17 +558,18 @@ EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);
 /*
  * Enter an RCU extended quiescent state, which can be either the
  * idle loop or adaptive-tickless usermode execution.
- *
- * We crowbar the ->dynticks_nmi_nesting field to zero to allow for
- * the possibility of usermode upcalls having messed up our count
- * of interrupt nesting level during the prior busy period.
  */
 static void rcu_eqs_enter(bool user)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
-	WARN_ON_ONCE(rdp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE);
-	WRITE_ONCE(rdp->dynticks_nmi_nesting, 0);
+	/* Entering usermode/idle from interrupt is not handled. These would
+	 * mean usermode upcalls or idle entry happened from interrupts. But,
+	 * reset the counter if we warn.
+	 */

Please either put the "/*" on its own line or use "//"-style comments.

I'll put "/*" on its own line.

quoted

 	WARN_ON_ONCE(rdp->dynticks_nmi_nesting <= 0);
 	WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs());
 
+	WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
+		   rdp->dynticks_nmi_nesting - 1);

This is problematic.  The +/-1 and +/-2 dance is specifically for NMIs, so...

This counter is deleted in the following patch so I hope its Ok to leave it
here for this one. I just kept it split into different patch to make
testing/review/development easier.

quoted

 	if (irq)
 		rcu_prepare_for_idle();

@@ -723,10 +728,6 @@ void rcu_irq_exit_irqson(void)
 /*
  * Exit an RCU extended quiescent state, which can be either the
  * idle loop or adaptive-tickless usermode execution.
- *
- * We crowbar the ->dynticks_nmi_nesting field to DYNTICK_IRQ_NONIDLE to
- * allow for the possibility of usermode upcalls messing up our count of
- * interrupt nesting level during the busy period that is just now starting.
  */
 static void rcu_eqs_exit(bool user)
 {

@@ -747,8 +748,13 @@ static void rcu_eqs_exit(bool user)
 	trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, rdp->dynticks);
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
 	WRITE_ONCE(rdp->dynticks_nesting, 1);
-	WARN_ON_ONCE(rdp->dynticks_nmi_nesting);
-	WRITE_ONCE(rdp->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
+
+	/* Exiting usermode/idle from interrupt is not handled. These would
+	 * mean usermode upcalls or idle exit happened from interrupts. But,
+	 * reset the counter if we warn.
+	 */
+	if (WARN_ON_ONCE(rdp->dynticks_nmi_nesting != 0))
+		WRITE_ONCE(rdp->dynticks_nmi_nesting, 0);

And here.  Plus this is adding a test and branch in the common case.
Given that the location being written to should be hot in the cache,
it is not clear that this is a win.

The next patch removes the branch itself and just has the warning.

quoted

 	WARN_ON_ONCE(rdp->dynticks_nmi_nesting < 0);
 
 	/*

@@ -826,16 +833,21 @@ static __always_inline void rcu_nmi_enter_common(bool irq)
 
 		incby = 1;
 	} else if (tick_nohz_full_cpu(rdp->cpu) &&
-		   rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE &&
-		   rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
+		   !rdp->dynticks_nmi_nesting && rdp->rcu_urgent_qs &&
+		   !rdp->rcu_forced_tick) {

OK.  Though you should be able to save a line by pulling the
"rdp->rcu_urgent_qs &&" onto the first line.

Fixed.

quoted

 		rdp->rcu_forced_tick = true;
 		tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
 	}
+

Not clear that the added blank line is a win, here or below.

Fixed,

thanks!

 - Joel

[snip]