Re: [tip/core/rcu, 05/21] rcu: Make rcu_gp_cleanup() more accurately predict need for new GP

[Paul E. McKenney]

On Thu, May 10, 2018 at 10:22:40AM -0700, Joel Fernandes wrote:
> On Thu, May 10, 2018 at 06:15:46AM -0700, Paul E. McKenney wrote:
> > On Thu, May 10, 2018 at 12:21:33AM -0700, Joel Fernandes wrote:
> > > Hi Paul,
> > > 
> > > On Sun, Apr 22, 2018 at 08:03:28PM -0700, Paul E. McKenney wrote:
> > > > Currently, rcu_gp_cleanup() scans the rcu_node tree in order to reset
> > > > state to reflect the end of the grace period.  It also checks to see
> > > > whether a new grace period is needed, but in a number of cases, rather
> > > > than directly cause the new grace period to be immediately started, it
> > > > instead leaves the grace-period-needed state where various fail-safes
> > > > can find it.  This works fine, but results in higher contention on the
> > > > root rcu_node structure's ->lock, which is undesirable, and contention
> > > > on that lock has recently become noticeable.
> > > > 
> > > > This commit therefore makes rcu_gp_cleanup() immediately start a new
> > > > grace period if there is any need for one.
> > > > 
> > > > It is quite possible that it will later be necessary to throttle the
> > > > grace-period rate, but that can be dealt with when and if.
> > > > 
> > > > Reported-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > ---
> > > >  kernel/rcu/tree.c        | 16 ++++++++++------
> > > >  kernel/rcu/tree.h        |  1 -
> > > >  kernel/rcu/tree_plugin.h | 17 -----------------
> > > >  3 files changed, 10 insertions(+), 24 deletions(-)
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index 497f139056c7..afc5e32f0da4 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -1763,14 +1763,14 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
> > > >   * Clean up any old requests for the just-ended grace period.  Also return
> > > >   * whether any additional grace periods have been requested.
> > > >   */
> > > > -static int rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
> > > > +static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
> > > >  {
> > > >  	int c = rnp->completed;
> > > > -	int needmore;
> > > > +	bool needmore;
> > > >  	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
> > > >  
> > > >  	need_future_gp_element(rnp, c) = 0;
> > > > -	needmore = need_future_gp_element(rnp, c + 1);
> > > > +	needmore = need_any_future_gp(rnp);
> > > >  	trace_rcu_future_gp(rnp, rdp, c,
> > > >  			    needmore ? TPS("CleanupMore") : TPS("Cleanup"));
> > > >  	return needmore;
> > > > @@ -2113,7 +2113,6 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
> > > >  {
> > > >  	unsigned long gp_duration;
> > > >  	bool needgp = false;
> > > > -	int nocb = 0;
> > > >  	struct rcu_data *rdp;
> > > >  	struct rcu_node *rnp = rcu_get_root(rsp);
> > > >  	struct swait_queue_head *sq;
> > > > @@ -2152,7 +2151,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
> > > >  		if (rnp == rdp->mynode)
> > > >  			needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
> > > >  		/* smp_mb() provided by prior unlock-lock pair. */
> > > > -		nocb += rcu_future_gp_cleanup(rsp, rnp);
> > > > +		needgp = rcu_future_gp_cleanup(rsp, rnp) || needgp;
> > > >  		sq = rcu_nocb_gp_get(rnp);
> > > >  		raw_spin_unlock_irq_rcu_node(rnp);
> > > >  		rcu_nocb_gp_cleanup(sq);
> > > > @@ -2162,13 +2161,18 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
> > > >  	}
> > > >  	rnp = rcu_get_root(rsp);
> > > >  	raw_spin_lock_irq_rcu_node(rnp); /* Order GP before ->completed update. */
> > > > -	rcu_nocb_gp_set(rnp, nocb);
> > > >  
> > > >  	/* Declare grace period done. */
> > > >  	WRITE_ONCE(rsp->completed, rsp->gpnum);
> > > >  	trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
> > > >  	rsp->gp_state = RCU_GP_IDLE;
> > > > +	/* Check for GP requests since above loop. */
> > > >  	rdp = this_cpu_ptr(rsp->rda);
> > > > +	if (need_any_future_gp(rnp)) {
> > > > +		trace_rcu_future_gp(rnp, rdp, rsp->completed - 1,
> > > > +				    TPS("CleanupMore"));
> > > > +		needgp = true;
> > > 
> > > Patch makes sense to me.
> > > 
> > > I didn't get the "rsp->completed - 1" bit in the call to trace_rcu_future_gp.
> > > The grace period that just completed is in rsp->completed. The future one
> > > should be completed + 1. What is meaning of the third argument 'c' to the
> > > trace event?
> > 
> > The thought was that the grace period must have been requested while
> > rsp->completed was one less than it is now.
> > 
> > In the current code, it uses rnp->gp_seq_needed, which is instead the grace
> > period that is being requested.
> 
> Oh ok, IIUC from the code, the 'c' parameter passed to trace_rcu_future_gp is
> the grace-period number in the future. Perhaps we should clarify this in the
> include/trace/events/rcu.h code what this parameter means. Probably
> 'future_gp' or something like that.
> 
> > > Also in rcu_future_gp_cleanup, we call:
> > > 	trace_rcu_future_gp(rnp, rdp, c,
> > > 			    needmore ? TPS("CleanupMore") : TPS("Cleanup"));
> > > For this case, in the final trace event record, rnp->completed and c will be
> > > the same, since c is set to rnp->completed before calling
> > > trace_rcu_future_gp. I was thinking they should be different, do you expect
> > > them to be the same?
> > 
> > Hmmm...  That does look a bit inconsistent.  And it currently uses
> > rnp->gp_seq instead of rnp->gp_seq_needed despite having the same
> > "CleanupMore" name.
> 
> Yes I was thinking in rcu_future_gp_cleanup, the call to trace_rcu_future_gp
> should be trace_rcu_future_gp(rnp, rdp, c + 1, needmore...);
> 
> This is because in rcu_future_gp_cleanup, c is set to rnp->completed. Just
> before this point rnp->completed was set to rsp->gpnum, which marks the end of
> the GP for the node. The next gp would be c + 1 right?
> 
> > Looks like a review of the calls to trace_rcu_this_gp() is in order.
> 
> Yes, I'll do some tracing and see if something else doesn't make sense to me
> and let you know.
> 
> > Or did you have suggestions for name/gp assocations for this trace
> > message type?
> 
> I think the name for this one is fine but also that "CleanupMore" sounds like
> more clean up is needed. It could be improved to "CleanupNeedgp" or
> "CleanupAndStart" or something like that.

Would you be willing to pick a name, check the grace-period numbers, and
send a patch relative to rcu/dev?

								Thanx, Paul