Re: Untested patch to recheck idle state for expedited grace periods

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

On Tue, Oct 11, 2016 at 09:15:40AM -0700, Paul E. McKenney wrote:
> On Tue, Oct 11, 2016 at 06:28:49AM -0700, Paul E. McKenney wrote:
> > Hello, Rik,
> > 
> > And it turns out that I did not in fact do the recheck at IPI time.
> > The (untested) patch below is an alleged fix.  Thoughts?
> 
> And it passes modest rcutorture testing, for whatever that might be
> worth.

How does it do at your end?  If I don't hear otherwise, I will assume
that it doesn't help and will mothball it.

							Thanx, Paul

> > ------------------------------------------------------------------------
> > 
> > commit e53e0b3e7b3c783962f9461bcb9aa8bc3e3a8688
> > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Date:   Tue Oct 11 06:09:59 2016 -0700
> > 
> >     rcu: Make expedited grace periods recheck dyntick idle state
> >     
> >     Expedited grace periods check dyntick-idle state, and avoid sending
> >     IPIs to idle CPUs, including those running guest OSes, and, on NOHZ_FULL
> >     kernels, nohz_full CPUs.  However, the kernel has been observed checking
> >     a CPU while it was non-idle, but sending the IPI after it has gone
> >     idle.  This commit therefore rechecks idle state immediately before
> >     sending the IPI, refraining from IPIing CPUs that have since gone idle.
> >     
> >     Reported-by: Rik van Riel <riel@redhat.com>
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index e99a5234d9ed..fe98dd24adf8 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -404,6 +404,7 @@ struct rcu_data {
> >  	atomic_long_t exp_workdone1;	/* # done by others #1. */
> >  	atomic_long_t exp_workdone2;	/* # done by others #2. */
> >  	atomic_long_t exp_workdone3;	/* # done by others #3. */
> > +	int exp_dynticks_snap;		/* Double-check need for IPI. */
> >  
> >  	/* 7) Callback offloading. */
> >  #ifdef CONFIG_RCU_NOCB_CPU
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index 24343eb87b58..d3053e99fdb6 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -358,8 +358,10 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
> >  			struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
> >  			struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> >  
> > +			rdp->exp_dynticks_snap =
> > +				atomic_add_return(0, &rdtp->dynticks);
> >  			if (raw_smp_processor_id() == cpu ||
> > -			    !(atomic_add_return(0, &rdtp->dynticks) & 0x1) ||
> > +			    !(rdp->exp_dynticks_snap & 0x1) ||
> >  			    !(rnp->qsmaskinitnext & rdp->grpmask))
> >  				mask_ofl_test |= rdp->grpmask;
> >  		}
> > @@ -377,9 +379,17 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
> >  		/* IPI the remaining CPUs for expedited quiescent state. */
> >  		for_each_leaf_node_possible_cpu(rnp, cpu) {
> >  			unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
> > +			struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
> > +			struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> > +
> >  			if (!(mask_ofl_ipi & mask))
> >  				continue;
> >  retry_ipi:
> > +			if (atomic_add_return(0, &rdtp->dynticks) !=
> > +			    rdp->exp_dynticks_snap) {
> > +				mask_ofl_test |= mask;
> > +				continue;
> > +			}
> >  			ret = smp_call_function_single(cpu, func, rsp, 0);
> >  			if (!ret) {
> >  				mask_ofl_ipi &= ~mask;