Re: [RFC][PATCH v3 6/6] rcu/tree: Use irq_work_queue_remote()

[Paul E. McKenney]

On Thu, Oct 29, 2020 at 10:15:34AM +0100, Peter Zijlstra wrote:
> On Wed, Oct 28, 2020 at 01:15:54PM -0700, Paul E. McKenney wrote:
> > On Wed, Oct 28, 2020 at 09:02:43PM +0100, Peter Zijlstra wrote:
> > > -#ifdef CONFIG_IRQ_WORK
> > > +		raw_spin_lock_rcu_node(rnp);
> > 
> > The caller of rcu_implicit_dynticks_qs() already holds this lock.
> > Please see the force_qs_rnp() function and its second call site,
> > to which rcu_implicit_dynticks_qs() is passed as an argument.
> 
> Like this then.

This does look plausible!  But I am sure that rcutorture will also
have an opinion.  ;-)

							Thanx, Paul

> ---
> Subject: rcu/tree: Use irq_work_queue_remote()
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Wed Oct 28 11:53:40 CET 2020
> 
> The effect of an self-IPI here would be setting rcu_iw_gp_seq to the
> value we just set it to (pointless) and clearing rcu_iw_pending, which
> we just set, so don't set it.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  kernel/rcu/tree.c |   10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1204,6 +1204,8 @@ static int rcu_implicit_dynticks_qs(stru
>  	bool *ruqp;
>  	struct rcu_node *rnp = rdp->mynode;
>  
> +	raw_lockdep_assert_held_rcu_node(rnp);
> +
>  	/*
>  	 * If the CPU passed through or entered a dynticks idle phase with
>  	 * no active irq/NMI handlers, then we can safely pretend that the CPU
> @@ -1308,14 +1310,14 @@ static int rcu_implicit_dynticks_qs(stru
>  			resched_cpu(rdp->cpu);
>  			WRITE_ONCE(rdp->last_fqs_resched, jiffies);
>  		}
> -#ifdef CONFIG_IRQ_WORK
>  		if (!rdp->rcu_iw_pending && rdp->rcu_iw_gp_seq != rnp->gp_seq &&
>  		    (rnp->ffmask & rdp->grpmask)) {
> -			rdp->rcu_iw_pending = true;
>  			rdp->rcu_iw_gp_seq = rnp->gp_seq;
> -			irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
> +			if (likely(rdp->cpu != smp_processor_id())) {
> +				rdp->rcu_iw_pending = true;
> +				irq_work_queue_remote(rdp->cpu, &rdp->rcu_iw);
> +			}
>  		}
> -#endif
>  	}
>  
>  	return 0;