Re: [PATCH rcu 04/18] rcu: Weaken ->dynticks accesses and updates

[Paul E. McKenney]

On Fri, Jul 30, 2021 at 01:56:41PM +0800, Boqun Feng wrote:
> On Thu, Jul 29, 2021 at 12:53:31PM +0200, Frederic Weisbecker wrote:
> > On Thu, Jul 29, 2021 at 03:58:04PM +0800, Boqun Feng wrote:
> > > > The following litmus test, also adapted from the one supplied off-list
> > > > by Frederic Weisbecker, models the RCU grace-period kthread detecting
> > > > a non-idle CPU that is concurrently transitioning to idle:
> > > > 
> > > > 	C dynticks-into-idle
> > > > 
> > > > 	{
> > > > 		DYNTICKS=1; (* Initially non-idle. *)
> > > > 	}
> > > > 
> > > > 	P0(int *X, int *DYNTICKS)
> > > > 	{
> > > > 		int dynticks;
> > > > 
> > > > 		// Non-idle.
> > > > 		WRITE_ONCE(*X, 1);
> > > > 		dynticks = READ_ONCE(*DYNTICKS);
> > > > 		smp_store_release(DYNTICKS, dynticks + 1);
> > > > 		smp_mb();
> > > 
> > > this smp_mb() is not needed, as we rely on the release-acquire pair to
> > > provide the ordering.
> > > 
> > > This means that if we use different implementations (one w/ smp_mb(),
> > > another w/o) rcu_dynticks_inc() for idle-to-nonidle and nonidle-to-idle,
> > > we could save a smp_mb(). Thoughts?
> > 
> > That's exactly what I wanted to propose but everybody was sober. Namely order
> > only the RCU read side critical sections before/after idle together:
> > 
> >      READ side critical section
> >      //enter idle
> >      //exit idle
> >      smp_mb()
> >      READ side critical section
> > 
> > instead of ordering the RCU read side critical section before idle - with the RCU
> > idle extended quiescent state - with the RCU read side critical section after idle:
> > 
> >      READ side critical section
> >      //enter idle
> >      smp_mb();
> >      //exit idle
> >      smp_mb()
> >      READ side critical section
> > 
> > So the side effect now is that if the write side waits for the reader to
> > report a quiescent state and scans its dynticks state and see it's not yet in
> > RCU idle mode, then later on when the read side enters in RCU idle mode we
> > expect it to see the write side updates.
> > But after the barrier removal the reader will only see the write side update
> > once we exit RCU idle mode.
> > 
> > So the following may happen:
> > 
> > 	P0(int *X, int *Y, int *DYNTICKS)
> > 	{
> > 		int y;
> > 
> > 		WRITE_ONCE(*X, 1);
> > 		smp_store_release(DYNTICKS, 1); // rcu_eqs_enter
> > 		//smp_mb() not there anymore
> > 		y = READ_ONCE(*Y);
> > 		smp_store_release(DYNTICKS, 2); // rcu_eqs_exit()
> > 		smp_mb();
> > 	}
> > 
> > 	P1(int *X, int *Y, int *DYNTICKS)
> > 	{
> > 		int x;
> > 		int dynticks;
> > 		
> > 		WRITE_ONCE(*Y, 1);
> > 		smp_mb();
> > 		dynticks = smp_load_acquire(DYNTICKS);
> > 		x = READ_ONCE(*X);
> > 	}
> > 
> > 	exists (1:x=0 /\ 0:y=0)
> > 
> 
> Thanks for the detailed explanation ;-)
> 
> > Theoretically it shouldn't matter because the RCU idle mode isn't
> > supposed to perform RCU reads. But theoretically again once a CPU
> 
> Right, in LOCKDEP=y kernel, rcu_read_lock_held() requires
> rcu_is_watching(), so rcu_dereference() is not allowed in idle mode,
> unless using RCU_NONIDLE() or rcu_irq_enter_irqson() to temporarily exit
> the idle mode.
> 
> > has reported a quiescent state, any further read is expected to see
> > the latest updates from the write side.
> 
> Yes, but in your above case, doesn't P0 already reach to a quiescent
> state even before WRITE_ONCE()? IOW, that case is similar to the
> following:
> 
> 	P0(int *X, int *Y)
> 	{
> 		// in QS
> 
> 		WRITE_ONCE(*X, 1);
> 		y = READ_ONCE(*Y);
> 	}
> 
> 	P1(int *X, int *Y)
> 	{
> 		WRITE_ONCE(*Y, 1);
> 		synchronize_rcu();
> 		x = READ_ONCE(*X);
> 	}
> 
> 	exists (1:x=0 /\ 0:y=0)
> 
> And RCU doesn't guarantee the READ_ONCE() on P0 sees the WRITE_ONCE() on
> P1.
> 
> > 
> > So I don't know what to think. In practice I believe it's not a big deal
> > because RCU idle mode code is usually a fragile path that just handles
> > cpuidle code to put the CPU in/out low power mode. But what about dragons...
> 
> My current thought is that if the cpuidle code requires some ordering
> with synchronize_rcu(), RCU_NONIDLE() should be used, and ordering can
> be guaranteed in this case (RCU_NONIDLE() has a rcu_eqs_exit() in it).
> Otherwise, it's a bug.
> 
> So looks like we can drop that smp_mb() in rcu_eqs_enter()? At least, we
> can say something in the doc to prevent people from relying on the
> ordering between normal reads in RCU idle mode and synchronize_rcu().
> 
> Thoughts?

Is there a benchmark that can show a system-level difference?  My
guess is that the realtime interrupt-latency and scheduler-latency
benchmarks would have the best chance of seeing this.

							Thanx, Paul