Re: [PATCH tip/core/rcu 3/6] rcu: Abstract dynticks extended quiescent state enter/exit operations

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Jan 16, 2017 at 03:34:20AM -0800, Paul E. McKenney wrote:
> On Sun, Jan 15, 2017 at 11:47:35PM -0800, Josh Triplett wrote:
> > On Sat, Jan 14, 2017 at 12:54:42AM -0800, Paul E. McKenney wrote:
> > > This commit is the third step towards full abstraction of all accesses
> > > to the ->dynticks counter, implementing the previously open-coded atomic
> > > add of 1 and entry checks in a new rcu_dynticks_eqs_enter() function, and
> > > the same but with exit checks in a new rcu_dynticks_eqs_exit() function.
> > > This abstraction will ease changes to the ->dynticks counter operation.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> > A couple of comments below.  With those addressed:
> > Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> > 
> > >  kernel/rcu/tree.c | 92 +++++++++++++++++++++++++++++++++++++++----------------
> > >  1 file changed, 66 insertions(+), 26 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 805d55ee0b2a..fc49e008963a 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -282,6 +282,65 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
> > >  };
> > >  
> > >  /*
> > > + * 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);
> > > +
> > > +	/*
> > > +	 * CPUs seeing atomic_inc() 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. */
> > > +	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > +		     atomic_read(&rdtp->dynticks) & 0x1);
> > > +}
> > > +
> > > +/*
> > > + * Record exit from an extended quiescent state.  This is only to be
> > > + * called from an extended quiescent state.
> > > + */
> > > +static void rcu_dynticks_eqs_exit(void)
> > > +{
> > > +	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > > +
> > > +	/*
> > > +	 * CPUs seeing atomic_inc() 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. */
> > > +	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > +		     !(atomic_read(&rdtp->dynticks) & 0x1));
> > > +}
> > > +
> > > +/*
> > > + * Reset the current CPU's ->dynticks counter to indicate that the
> > > + * newly onlined CPU is no longer in an extended quiescent state.
> > > + * This will either leave the counter unchanged, or increment it
> > > + * to the next non-quiescent value.
> > > + *
> > > + * The non-atomic test/increment sequence works because the upper bits
> > > + * of the ->dynticks counter are manipulated only by the corresponding CPU,
> > > + * or when the corresponding CPU is offline.
> > > + */
> > > +static void rcu_dynticks_eqs_online(void)
> > > +{
> > > +	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > > +
> > > +	if (atomic_read(&rdtp->dynticks) & 0x1)
> > > +		return;
> > > +	atomic_add(0x1, &rdtp->dynticks);
> > > +}
> > > +
> > > +/*
> > >   * Snapshot the ->dynticks counter with full ordering so as to allow
> > >   * stable comparison of this counter with past and future snapshots.
> > >   */
> > > @@ -693,7 +752,7 @@ static void rcu_eqs_enter_common(long long oldval, bool user)
> > >  {
> > >  	struct rcu_state *rsp;
> > >  	struct rcu_data *rdp;
> > > -	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > > +	struct rcu_dynticks __maybe_unused *rdtp = this_cpu_ptr(&rcu_dynticks);
> > 
> > Rather than marking a local variable as __maybe_unused (such that the
> > compiler can no longer help detect it as unused), could you move it into
> > the portion of the function that uses it, so that if reached, it'll
> > always get used?
> > 
> > >  	trace_rcu_dyntick(TPS("Start"), oldval, rdtp->dynticks_nesting);
> 
> Its only use is in the above event trace, which can be disabled via
> CONFIG_RCU_TRACE=n.  I could put the definition of rdtp under #ifdef,
> but this seems ugly.  I could eliminate the variable, substituting
> the initialization for rdtp in the event trace, but that would make
> for a very long line, or an odd line break.

For the trace_rcu_dyntick calls, you could create a small static inline
helper to do the tracing, eliminating the third argument.  Or you could
create a helper that returns
this_cpu_ptr(&rcu_dynticks)->dynticks_nesting.  Either way would work.

Or, if you prefer, you could wrap the variable
declaration/initialization in RCU_TRACE() rather than adding
__maybe_unused.

> > >  	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > @@ -712,12 +771,7 @@ static void rcu_eqs_enter_common(long long oldval, bool user)
> > >  		do_nocb_deferred_wakeup(rdp);
> > >  	}
> > >  	rcu_prepare_for_idle();
> > > -	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
> > > -	smp_mb__before_atomic();  /* See above. */
> > > -	atomic_inc(&rdtp->dynticks);
> > > -	smp_mb__after_atomic();  /* Force ordering with next sojourn. */
> > > -	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > -		     atomic_read(&rdtp->dynticks) & 0x1);
> > > +	rcu_dynticks_eqs_enter();
> > >  	rcu_dynticks_task_enter();
> > >  
> > >  	/*
> > > @@ -846,15 +900,10 @@ void rcu_irq_exit_irqson(void)
> > >   */
> > >  static void rcu_eqs_exit_common(long long oldval, int user)
> > >  {
> > > -	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > > +	struct rcu_dynticks __maybe_unused *rdtp = this_cpu_ptr(&rcu_dynticks);
> > 
> > Same comment as above.
> > 
> > >  	rcu_dynticks_task_exit();
> > > -	smp_mb__before_atomic();  /* Force ordering w/previous sojourn. */
> > > -	atomic_inc(&rdtp->dynticks);
> > > -	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
> > > -	smp_mb__after_atomic();  /* See above. */
> > > -	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > -		     !(atomic_read(&rdtp->dynticks) & 0x1));
> > > +	rcu_dynticks_eqs_exit();
> > >  	rcu_cleanup_after_idle();
> > >  	trace_rcu_dyntick(TPS("End"), oldval, rdtp->dynticks_nesting);
> 
> This one is used here, at the top level of the function, and a few
> lines down.  I see the same options available.  Thoughts?

In the fourth patch, you introduce a helper that replaces the use in
atomic_read below.  That leaves only the tracing use, where the same
helper as above would apply.

> 							Thanx, Paul
> 
> > >  	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > @@ -1001,11 +1050,7 @@ void rcu_nmi_enter(void)
> > >  	 * period (observation due to Andy Lutomirski).
> > >  	 */
> > >  	if (!(atomic_read(&rdtp->dynticks) & 0x1)) {
> > > -		smp_mb__before_atomic();  /* Force delay from prior write. */
> > > -		atomic_inc(&rdtp->dynticks);
> > > -		/* atomic_inc() before later RCU read-side crit sects */
> > > -		smp_mb__after_atomic();  /* See above. */
> > > -		WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
> > > +		rcu_dynticks_eqs_exit();
> > >  		incby = 1;
> > >  	}
> > >  	rdtp->dynticks_nmi_nesting += incby;
> > > @@ -1043,11 +1088,7 @@ void rcu_nmi_exit(void)
> > >  
> > >  	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
> > >  	rdtp->dynticks_nmi_nesting = 0;
> > > -	/* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
> > > -	smp_mb__before_atomic();  /* See above. */
> > > -	atomic_inc(&rdtp->dynticks);
> > > -	smp_mb__after_atomic();  /* Force delay to next write. */
> > > -	WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
> > > +	rcu_dynticks_eqs_enter();
> > >  }
> > >  
> > >  /**
> > > @@ -3800,8 +3841,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
> > >  		init_callback_list(rdp);  /* Re-enable callbacks on this CPU. */
> > >  	rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
> > >  	rcu_sysidle_init_percpu_data(rdp->dynticks);
> > > -	atomic_set(&rdp->dynticks->dynticks,
> > > -		   (atomic_read(&rdp->dynticks->dynticks) & ~0x1) + 1);
> > > +	rcu_dynticks_eqs_online();
> > >  	raw_spin_unlock_rcu_node(rnp);		/* irqs remain disabled. */
> > >  
> > >  	/*
> > > -- 
> > > 2.5.2
> > > 
> > 
>