Re: need_heavy_qs flag for PREEMPT=y kernels

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Mon, Aug 12, 2019 at 09:02:49PM -0400, Joel Fernandes wrote:
> On Mon, Aug 12, 2019 at 04:01:38PM -0700, Paul E. McKenney wrote:

[ . . . ]

> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 8c494a692728..ad906d6a74fb 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -651,6 +651,12 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
> >  	 */
> >  	if (rdp->dynticks_nmi_nesting != 1) {
> >  		trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, rdp->dynticks);
> > +		if (tick_nohz_full_cpu(rdp->cpu) &&
> > +		    rdp->dynticks_nmi_nesting == 2 &&
> > +		    rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
> > +			rdp->rcu_forced_tick = true;
> > +			tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU);
> > +		}
> 
> 
> Instead of checking dynticks_nmi_nesting == 2 in rcu_nmi_exit_common(), can
> we do the tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU)  from
> rcu_nmi_enter_common() ? We could add this code there, under the "if
> (rcu_dynticks_curr_cpu_in_eqs())".

This would need to go in an "else" clause, correct?  But there would still
want to be a check for interrupt from base level (which would admittedly
be an equality comparison with zero) and we would also still need to check
for rdp->rcu_urgent_qs && !rdp->rcu_forced_tick.

Still, an equal-zero comparison is probably going to be a bit cheaper than
an equals-two comparison, and this is on the interrupt-entry fastpath,
so this change is likely worth making.  Good call!!!

							Thanx, Paul

> I will test this patch tomorrow and let you know how it goes.
> 
> thanks,
> 
>  - Joel
> 
> 
> 
> 
> >  		WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
> >  			   rdp->dynticks_nmi_nesting - 2);
> >  		return;
> > @@ -886,6 +892,16 @@ void rcu_irq_enter_irqson(void)
> >  	local_irq_restore(flags);
> >  }
> >  
> > +/*
> > + * If the scheduler-clock interrupt was enabled on a nohz_full CPU
> > + * in order to get to a quiescent state, disable it.
> > + */
> > +void rcu_disable_tick_upon_qs(struct rcu_data *rdp)
> > +{
> > +	if (tick_nohz_full_cpu(rdp->cpu) && rdp->rcu_forced_tick)
> > +		tick_dep_clear_cpu(rdp->cpu, TICK_DEP_MASK_RCU);
> > +}
> > +
> >  /**
> >   * rcu_is_watching - see if RCU thinks that the current CPU is not idle
> >   *
> > @@ -1980,6 +1996,7 @@ rcu_report_qs_rdp(int cpu, struct rcu_data *rdp)
> >  		if (!offloaded)
> >  			needwake = rcu_accelerate_cbs(rnp, rdp);
> >  
> > +		rcu_disable_tick_upon_qs(rdp);
> >  		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
> >  		/* ^^^ Released rnp->lock */
> >  		if (needwake)
> > @@ -2269,6 +2286,7 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
> >  	int cpu;
> >  	unsigned long flags;
> >  	unsigned long mask;
> > +	struct rcu_data *rdp;
> >  	struct rcu_node *rnp;
> >  
> >  	rcu_for_each_leaf_node(rnp) {
> > @@ -2293,8 +2311,10 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
> >  		for_each_leaf_node_possible_cpu(rnp, cpu) {
> >  			unsigned long bit = leaf_node_cpu_bit(rnp, cpu);
> >  			if ((rnp->qsmask & bit) != 0) {
> > -				if (f(per_cpu_ptr(&rcu_data, cpu)))
> > -					mask |= bit;
> > +				rdp = per_cpu_ptr(&rcu_data, cpu);
> > +				if (f(rdp))
> > +					rcu_disable_tick_upon_qs(rdp);
> > +				mask |= bit;
> >  			}
> >  		}
> >  		if (mask != 0) {
> > @@ -2322,7 +2342,7 @@ void rcu_force_quiescent_state(void)
> >  	rnp = __this_cpu_read(rcu_data.mynode);
> >  	for (; rnp != NULL; rnp = rnp->parent) {
> >  		ret = (READ_ONCE(rcu_state.gp_flags) & RCU_GP_FLAG_FQS) ||
> > -		      !raw_spin_trylock(&rnp->fqslock);
> > +		       !raw_spin_trylock(&rnp->fqslock);
> >  		if (rnp_old != NULL)
> >  			raw_spin_unlock(&rnp_old->fqslock);
> >  		if (ret)
> > @@ -2855,7 +2875,7 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
> >  {
> >  	if (atomic_dec_and_test(&rcu_state.barrier_cpu_count)) {
> >  		rcu_barrier_trace(TPS("LastCB"), -1,
> > -				   rcu_state.barrier_sequence);
> > +				  rcu_state.barrier_sequence);
> >  		complete(&rcu_state.barrier_completion);
> >  	} else {
> >  		rcu_barrier_trace(TPS("CB"), -1, rcu_state.barrier_sequence);
> > @@ -2879,7 +2899,7 @@ static void rcu_barrier_func(void *unused)
> >  	} else {
> >  		debug_rcu_head_unqueue(&rdp->barrier_head);
> >  		rcu_barrier_trace(TPS("IRQNQ"), -1,
> > -				   rcu_state.barrier_sequence);
> > +				  rcu_state.barrier_sequence);
> >  	}
> >  	rcu_nocb_unlock(rdp);
> >  }
> > @@ -2906,7 +2926,7 @@ void rcu_barrier(void)
> >  	/* Did someone else do our work for us? */
> >  	if (rcu_seq_done(&rcu_state.barrier_sequence, s)) {
> >  		rcu_barrier_trace(TPS("EarlyExit"), -1,
> > -				   rcu_state.barrier_sequence);
> > +				  rcu_state.barrier_sequence);
> >  		smp_mb(); /* caller's subsequent code after above check. */
> >  		mutex_unlock(&rcu_state.barrier_mutex);
> >  		return;
> > @@ -2938,11 +2958,11 @@ void rcu_barrier(void)
> >  			continue;
> >  		if (rcu_segcblist_n_cbs(&rdp->cblist)) {
> >  			rcu_barrier_trace(TPS("OnlineQ"), cpu,
> > -					   rcu_state.barrier_sequence);
> > +					  rcu_state.barrier_sequence);
> >  			smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
> >  		} else {
> >  			rcu_barrier_trace(TPS("OnlineNQ"), cpu,
> > -					   rcu_state.barrier_sequence);
> > +					  rcu_state.barrier_sequence);
> >  		}
> >  	}
> >  	put_online_cpus();
> > @@ -3168,6 +3188,7 @@ void rcu_cpu_starting(unsigned int cpu)
> >  	rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
> >  	rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
> >  	if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */
> > +		rcu_disable_tick_upon_qs(rdp);
> >  		/* Report QS -after- changing ->qsmaskinitnext! */
> >  		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
> >  	} else {
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index c612f306fe89..055c31781d3a 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -181,6 +181,7 @@ struct rcu_data {
> >  	atomic_t dynticks;		/* Even value for idle, else odd. */
> >  	bool rcu_need_heavy_qs;		/* GP old, so heavy quiescent state! */
> >  	bool rcu_urgent_qs;		/* GP old need light quiescent state. */
> > +	bool rcu_forced_tick;		/* Forced tick to provide QS. */
> >  #ifdef CONFIG_RCU_FAST_NO_HZ
> >  	bool all_lazy;			/* All CPU's CBs lazy at idle start? */
> >  	unsigned long last_accelerate;	/* Last jiffy CBs were accelerated. */