Re: task isolation discussion at Linux Plumbers

[Paul E. McKenney]

On Wed, Nov 09, 2016 at 08:52:13PM -0800, Paul E. McKenney wrote:
> On Wed, Nov 09, 2016 at 05:44:02PM -0800, Andy Lutomirski wrote:
> > On Wed, Nov 9, 2016 at 9:38 AM, Paul E. McKenney
> > <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > 
> > Are you planning on changing rcu_nmi_enter()?  It would make it easier
> > to figure out how they interact if I could see the code.
> 
> It already calls rcu_dynticks_eqs_exit(), courtesy of the earlier
> consolidation patches.
> 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index dbf20b058f48..342c8ee402d6 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > 
> > 
> > >  /*
> > > @@ -305,17 +318,22 @@ static void rcu_dynticks_eqs_enter(void)
> > >  static void rcu_dynticks_eqs_exit(void)
> > >  {
> > >         struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > > +       int seq;
> > >
> > >         /*
> > > -        * CPUs seeing atomic_inc() must see prior idle sojourns,
> > > +        * CPUs seeing atomic_inc_return() must see prior idle sojourns,
> > >          * and we also must force ordering with the next RCU read-side
> > >          * critical section.
> > >          */
> > > -       smp_mb__before_atomic(); /* See above. */
> > > -       atomic_inc(&rdtp->dynticks);
> > > -       smp_mb__after_atomic(); /* See above. */
> > > +       seq = atomic_inc_return(&rdtp->dynticks);
> > >         WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > -                    !(atomic_read(&rdtp->dynticks) & 0x1));
> > > +                    !(seq & RCU_DYNTICK_CTRL_CTR));
> > 
> > I think there's still a race here.  Suppose we're running this code on
> > cpu n and...
> > 
> > > +       if (seq & RCU_DYNTICK_CTRL_MASK) {
> > > +               rcu_eqs_special_exit();
> > > +               /* Prefer duplicate flushes to losing a flush. */
> > > +               smp_mb__before_atomic(); /* NMI safety. */
> > 
> > ... another CPU changes the page tables and calls rcu_eqs_special_set(n) here.
> 
> But then rcu_eqs_special_set() will return false because we already
> exited the extended quiescent state at the atomic_inc_return() above.
> 
> That should tell the caller to send an IPI.
> 
> > That CPU expects that we will flush prior to continuing, but we won't.
> > Admittedly it's highly unlikely that any stale TLB entries would be
> > created yet, but nothing rules it out.
> 
> That said, 0day is having some heartburn from this, so I must have broken
> something somewhere.  My own tests of course complete just fine...
> 
> > > +               atomic_and(~RCU_DYNTICK_CTRL_MASK, &rdtp->dynticks);
> > > +       }
> > 
> > Maybe the way to handle it is something like:
> > 
> > this_cpu_write(rcu_nmi_needs_eqs_special, 1);
> > barrier();
> > 
> > /* NMI here will call rcu_eqs_special_exit() regardless of the value
> > in dynticks */
> > 
> > atomic_and(...);
> > smp_mb__after_atomic();
> > rcu_eqs_special_exit();
> > 
> > barrier();
> > this_cpu_write(rcu_nmi_needs_eqs_special, 0);
> > 
> > 
> > Then rcu_nmi_enter() would call rcu_eqs_special_exit() if the dynticks
> > bit is set *or* rcu_nmi_needs_eqs_special is set.
> > 
> > Does that make sense?
> 
> I believe that rcu_eqs_special_set() returning false covers this, but
> could easily be missing something.

And fixing a couple of stupid errors might help.  Probably more where
those came from...

							Thanx, Paul

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

commit 0ea13930e9a89b1741897f5af308692eab08d9e8
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date:   Tue Nov 8 14:25:21 2016 -0800

    rcu: Maintain special bits at bottom of ->dynticks counter
    
    Currently, IPIs are used to force other CPUs to invalidate their TLBs
    in response to a kernel virtual-memory mapping change.  This works, but
    degrades both battery lifetime (for idle CPUs) and real-time response
    (for nohz_full CPUs), and in addition results in unnecessary IPIs due to
    the fact that CPUs executing in usermode are unaffected by stale kernel
    mappings.  It would be better to cause a CPU executing in usermode to
    wait until it is entering kernel mode to do the flush, first to avoid
    interrupting usemode tasks and second to handle multiple flush requests
    with a single flush in the case of a long-running user task.
    
    This commit therefore reserves a bit at the bottom of the ->dynticks
    counter, which is checked upon exit from extended quiescent states.
    If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is
    invoked, which, if not supplied, is an empty single-pass do-while loop.
    If this bottom bit is set on -entry- to an extended quiescent state,
    then a WARN_ON_ONCE() triggers.
    
    This bottom bit may be set using a new rcu_eqs_special_set() function,
    which returns true if the bit was set, or false if the CPU turned
    out to not be in an extended quiescent state.  Please note that this
    function refuses to set the bit for a non-nohz_full CPU when that CPU
    is executing in usermode because usermode execution is tracked by RCU
    as a dyntick-idle extended quiescent state only for nohz_full CPUs.
    
    Reported-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index 4f9b2fa2173d..7232d199a81c 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -33,6 +33,11 @@ static inline int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
 	return 0;
 }
 
+static inline bool rcu_eqs_special_set(int cpu)
+{
+	return false;  /* Never flag non-existent other CPUs! */
+}
+
 static inline unsigned long get_state_synchronize_rcu(void)
 {
 	return 0;
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index dbf20b058f48..c1e3d4a333b2 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -279,23 +279,36 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
 };
 
 /*
+ * Steal a bit from the bottom of ->dynticks for idle entry/exit
+ * control.  Initially this is for TLB flushing.
+ */
+#define RCU_DYNTICK_CTRL_MASK 0x1
+#define RCU_DYNTICK_CTRL_CTR  (RCU_DYNTICK_CTRL_MASK + 1)
+#ifndef rcu_eqs_special_exit
+#define rcu_eqs_special_exit() do { } while (0)
+#endif
+
+/*
  * Record entry into an extended quiescent state.  This is only to be
  * called when not already in an extended quiescent state.
  */
 static void rcu_dynticks_eqs_enter(void)
 {
 	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
+	int seq;
 
 	/*
-	 * CPUs seeing atomic_inc() must see prior RCU read-side critical
-	 * sections, and we also must force ordering with the next idle
-	 * sojourn.
+	 * CPUs seeing atomic_inc_return() must see prior RCU read-side
+	 * critical sections, and we also must force ordering with the
+	 * next idle sojourn.
 	 */
-	smp_mb__before_atomic(); /* See above. */
-	atomic_inc(&rdtp->dynticks);
-	smp_mb__after_atomic(); /* See above. */
+	seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);
+	/* Better be in an extended quiescent state! */
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
+		     (seq & RCU_DYNTICK_CTRL_CTR));
+	/* Better not have special action (TLB flush) pending! */
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
-		     atomic_read(&rdtp->dynticks) & 0x1);
+		     (seq & RCU_DYNTICK_CTRL_MASK));
 }
 
 /*
@@ -305,17 +318,22 @@ static void rcu_dynticks_eqs_enter(void)
 static void rcu_dynticks_eqs_exit(void)
 {
 	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
+	int seq;
 
 	/*
-	 * CPUs seeing atomic_inc() must see prior idle sojourns,
+	 * CPUs seeing atomic_inc_return() must see prior idle sojourns,
 	 * and we also must force ordering with the next RCU read-side
 	 * critical section.
 	 */
-	smp_mb__before_atomic(); /* See above. */
-	atomic_inc(&rdtp->dynticks);
-	smp_mb__after_atomic(); /* See above. */
+	seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
-		     !(atomic_read(&rdtp->dynticks) & 0x1));
+		     !(seq & RCU_DYNTICK_CTRL_CTR));
+	if (seq & RCU_DYNTICK_CTRL_MASK) {
+		rcu_eqs_special_exit();
+		/* Prefer duplicate flushes to losing a flush. */
+		smp_mb__before_atomic(); /* NMI safety. */
+		atomic_and(~RCU_DYNTICK_CTRL_MASK, &rdtp->dynticks);
+	}
 }
 
 /*
@@ -326,7 +344,7 @@ int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
 {
 	int snap = atomic_add_return(0, &rdtp->dynticks);
 
-	return snap;
+	return snap & ~RCU_DYNTICK_CTRL_MASK;
 }
 
 /*
@@ -335,7 +353,7 @@ int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
  */
 static bool rcu_dynticks_in_eqs(int snap)
 {
-	return !(snap & 0x1);
+	return !(snap & RCU_DYNTICK_CTRL_CTR);
 }
 
 /*
@@ -355,10 +373,33 @@ static bool rcu_dynticks_in_eqs_since(struct rcu_dynticks *rdtp, int snap)
 static void rcu_dynticks_momentary_idle(void)
 {
 	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
-	int special = atomic_add_return(2, &rdtp->dynticks);
+	int special = atomic_add_return(2 * RCU_DYNTICK_CTRL_CTR,
+					&rdtp->dynticks);
 
 	/* It is illegal to call this from idle state. */
-	WARN_ON_ONCE(!(special & 0x1));
+	WARN_ON_ONCE(!(special & RCU_DYNTICK_CTRL_CTR));
+}
+
+/*
+ * Set the special (bottom) bit of the specified CPU so that it
+ * will take special action (such as flushing its TLB) on the
+ * next exit from an extended quiescent state.  Returns true if
+ * the bit was successfully set, or false if the CPU was not in
+ * an extended quiescent state.
+ */
+bool rcu_eqs_special_set(int cpu)
+{
+	int old;
+	int new;
+	struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
+
+	do {
+		old = atomic_read(&rdtp->dynticks);
+		if (old & RCU_DYNTICK_CTRL_CTR)
+			return false;
+		new = old | RCU_DYNTICK_CTRL_MASK;
+	} while (atomic_cmpxchg(&rdtp->dynticks, old, new) != old);
+	return true;
 }
 
 DEFINE_PER_CPU_SHARED_ALIGNED(unsigned long, rcu_qs_ctr);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 3b953dcf6afc..7dcdd59d894c 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -596,6 +596,7 @@ extern struct rcu_state rcu_preempt_state;
 #endif /* #ifdef CONFIG_PREEMPT_RCU */
 
 int rcu_dynticks_snap(struct rcu_dynticks *rdtp);
+bool rcu_eqs_special_set(int cpu);
 
 #ifdef CONFIG_RCU_BOOST
 DECLARE_PER_CPU(unsigned int, rcu_cpu_kthread_status);