linux-kernel - [RFC 1/3] rcu: Expedite the rcu quiescent state reporting if help needed

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Message-Id: <20190701040415.219001-1-joel@joelfernandes.org>
Date:   Mon,  1 Jul 2019 00:04:13 -0400
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>, kernel-team@...roid.com,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kselftest@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>, rcu@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Shuah Khan <shuah@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [RFC 1/3] rcu: Expedite the rcu quiescent state reporting if help needed

The t->rcu_read_unlock_special union's need_qs bit can be set by the
scheduler tick (in rcu_flavor_sched_clock_irq) to indicate that help is
needed from the rcu_read_unlock path. When this help arrives however, we
can do better to speed up the quiescent state reporting which if
rcu_read_unlock_special::need_qs is set might be quite urgent. Make use
of this information in deciding when to do heavy-weight softirq raising
where possible.

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 kernel/rcu/tree_plugin.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c588ef98efd3..bff6410fac06 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -622,7 +622,8 @@ static void rcu_read_unlock_special(struct task_struct *t)
 		t->rcu_read_unlock_special.b.exp_hint = false;
 		exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) ||
 		      (rdp->grpmask & rnp->expmask) ||
-		      tick_nohz_full_cpu(rdp->cpu);
+		      tick_nohz_full_cpu(rdp->cpu)  ||
+		      t->rcu_read_unlock_special.b.need_qs;
 		// Need to defer quiescent state until everything is enabled.
 		if (irqs_were_disabled && use_softirq &&
 		    (in_interrupt() ||
-- 
2.22.0.410.gd8fdbe21b5-goog