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

[Joel Fernandes <joel@joelfernandes.org>]

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".

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;
>