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From:   "Joel Fernandes (Google)" <joel@joelfernandes.org>
To:     linux-kernel@vger.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Ingo Molnar <mingo@redhat.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E. McKenney" <paulmck@kernel.org>, rcu@vger.kernel.org,
        Steven Rostedt <rostedt@goodmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Subject: [PATCH 2/3] rcu/trace: Use gp_seq_req in acceleration's rcu_grace_period tracepoint
Date:   Thu, 18 Jun 2020 21:36:40 -0400
Message-Id: <20200619013641.63453-2-joel@joelfernandes.org>
In-Reply-To: <20200619013641.63453-1-joel@joelfernandes.org>
References: <20200619013641.63453-1-joel@joelfernandes.org>
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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 gp_seq_req instead as it allows one to
reason about how the acceleration works.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/rcu/tree.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9ef68dd249e1d..d0988a1c1079d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1464,9 +1464,10 @@ 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->gp_seq, TPS("AccWaitCB"));
+		trace_rcu_grace_period(rcu_state.name, gp_seq_req, TPS("AccWaitCB"));
 	else
-		trace_rcu_grace_period(rcu_state.name, rdp->gp_seq, TPS("AccReadyCB"));
+		trace_rcu_grace_period(rcu_state.name, gp_seq_req, TPS("AccReadyCB"));
+
 	return ret;
 }
 
-- 
2.27.0.111.gc72c7da667-goog