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

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Message-ID: <20200618222721.GA2723@paulmck-ThinkPad-P72>
Date:   Thu, 18 Jun 2020 15:27:21 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.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 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.

							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
>