Re: [PATCH RFC 8/8] rcu: Fix cpustart tracepoint gp_seq number

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, May 14, 2018 at 01:33:20PM -0700, Paul E. McKenney wrote:
> On Sun, May 13, 2018 at 08:15:41PM -0700, Joel Fernandes (Google) wrote:
> > cpustart shows a stale gp_seq. This is because rdp->gp_seq is updated
> > only at the end of the __note_gp_changes function. For this reason, use
> > rnp->gp_seq instead. I believe we can't update rdp->gp_seq too early so
> > lets just use the gp_seq from rnp instead.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> 
> I took this one with the usual commit-log update.  Please take a look!

Looks good, thanks!

- Joel


> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 3e14c6a5225d201776edd9a96d9c9b4435855446
> Author: Joel Fernandes (Google) <joel@joelfernandes.org>
> Date:   Sun May 13 20:15:41 2018 -0700
> 
>     rcu: Fix cpustart tracepoint gp_seq number
>     
>     The "cpustart" trace event shows a stale gp_seq. This is because it uses
>     rdp->gp_seq, which is updated only at the end of the __note_gp_changes()
>     function. This commit therefore instead uses rnp->gp_seq.
>     
>     An alternative fix would be to update rdp->gp_seq earlier, but this would
>     break RCU's detection of the beginning of a new-to-this-CPU grace period.
>     
>     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 de6447dd73de..23b855f5c5cb 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1796,7 +1796,7 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
>  		 * set up to detect a quiescent state, otherwise don't
>  		 * go looking for one.
>  		 */
> -		trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("cpustart"));
> +		trace_rcu_grace_period(rsp->name, rnp->gp_seq, TPS("cpustart"));
>  		need_gp = !!(rnp->qsmask & rdp->grpmask);
>  		rdp->cpu_no_qs.b.norm = need_gp;
>  		rdp->rcu_qs_ctr_snap = __this_cpu_read(rcu_dynticks.rcu_qs_ctr);
>