Re: [PATCH RFC 7/8] rcu: trace CleanupMore condition only if needed

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

On Mon, May 14, 2018 at 06:01:31PM -0700, Joel Fernandes wrote:
> On Mon, May 14, 2018 at 12:20:28PM -0700, Paul E. McKenney wrote:
> > On Sun, May 13, 2018 at 08:15:40PM -0700, Joel Fernandes (Google) wrote:
> > > Currently the tree RCU clean up code records a CleanupMore trace event
> > > even if the GP was already in progress. This makes CleanupMore show up
> > > twice for no reason. Avoid it.
> > > 
> > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > 
> > Good catch, and I applied this patch.  I did rework the commit log
> > a bit, so please look it over to make sure I didn't mess it up.
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 52c4e689efd975f5383895b1bc1b91bc90fdd372
> > Author: Joel Fernandes (Google) <joel@joelfernandes.org>
> > Date:   Sun May 13 20:15:40 2018 -0700
> > 
> >     rcu: Produce last "CleanupMore" trace only if late-breaking request
> >     
> >     Currently the tree RCU clean-up code records a "CleanupMore" trace
> >     event in response to late-arriving grace-period requests even if the
> >     grace period was already requested. This makes "CleanupMore" show up an
> >     extra time (in addition to once for each rcu_node structure that was
> >     previously marked with the request) for no good reason.  This commit
> >     therefore avoids emitting this trace message unless the only if the only
> >     request for this next grace period arrived during or after the cleanup
> >     scan of the rcu_node structures.
> 
> Yes, this is fine except "unless the only if the only" should be "unless the".

I did update this after sending, and I still have "the the".  Will fix on
next rebase.  As my daughter recently reminded me, the Law of Conservation
of Bugs.  ;-)

							Thanx, Paul

> thanks,
> 
> - Joel
> 
> >     
> >     Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 8063a0478870..de6447dd73de 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2072,7 +2072,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
> >  	rsp->gp_state = RCU_GP_IDLE;
> >  	/* Check for GP requests since above loop. */
> >  	rdp = this_cpu_ptr(rsp->rda);
> > -	if (ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
> > +	if (!needgp && ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
> >  		trace_rcu_this_gp(rnp, rdp, rnp->gp_seq_needed,
> >  				  TPS("CleanupMore"));
> >  		needgp = true;
> > 
>