linux-kernel - Re: [PATCH 5/7] rcu/trace: Use rsp's gp_seq in acceleration's rcu_grace

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Date:   Thu, 18 Jun 2020 19:54:44 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Marco Elver <elver@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: Re: [PATCH 5/7] rcu/trace: Use rsp's gp_seq in acceleration's
 rcu_grace_period tracepoint

On Thu, Jun 18, 2020 at 03:27:21PM -0700, Paul E. McKenney wrote:
> On Thu, Jun 18, 2020 at 04:29:53PM -0400, Joel Fernandes (Google) wrote:
> > During acceleration of CB, the rsp's gp_seq is rcu_seq_snap'd. This is
> > the value used for acceleration - it is the value of gp_seq at which it
> > is safe the execute all callbacks in the callback list.
> > 
> > The rdp's gp_seq is not very useful for this scenario. Make
> > rcu_grace_period report the rsp's gp_seq instead as it allows one to
> > reason about how the acceleration works.
> 
> Good catch, but please instead trace the gp_seq_req local variable.

Oh sure, I will do that. Actually I was planning to, but was wondering if it
would be inconsistent with other trace_rcu_grace_period() invocations which
pass a real gp_seq (one that is as old as rcu_state.gp_seq). But agreed, this
is better since the acceleration uses it.

thanks,

 - Joel

> 							Thanx, Paul
> 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> >  kernel/rcu/tree.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 81df1b837dd9d..c3bae7a83d792 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1437,9 +1437,9 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
> >  
> >  	/* Trace depending on how much we were able to accelerate. */
> >  	if (rcu_segcblist_restempty(&rdp->cblist, RCU_WAIT_TAIL))
> > -		trace_rcu_grace_period(rcu_state.name, "rdp", rdp->gp_seq, TPS("AccWaitCB"));
> > +		trace_rcu_grace_period(rcu_state.name, "rsp", rcu_state.gp_seq, TPS("AccWaitCB"));
> >  	else
> > -		trace_rcu_grace_period(rcu_state.name, "rdp", rdp->gp_seq, TPS("AccReadyCB"));
> > +		trace_rcu_grace_period(rcu_state.name, "rsp", rcu_state.gp_seq, TPS("AccReadyCB"));
> >  
> >  	/* Count CBs for tracing. */
> >  	rcu_segcblist_countseq(&rdp->cblist, cbs, gps);
> > -- 
> > 2.27.0.111.gc72c7da667-goog
> >