linux-kernel - [PATCH tip/core/rcu 07/12] rcu: Avoid self-IPI in sync_rcu_exp_select_node

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Date:   Thu, 30 May 2019 07:52:24 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 07/12] rcu: Avoid self-IPI in sync_rcu_exp_select_node_cpus()

Although sync_rcu_exp_select_node_cpus() treats the current CPU as being
in a quiescent state, it might well migrate to some other CPU before
reaching the smp_call_function_single(), which could then result in an
unnecessary simulated self-IPI.  This commit therefore instead simply
refuses to invoke smp_call_function_single() on the current CPU, which
causes the later rcu_report_exp_cpu_mult() to report this CPU's quiescent
state with less overhead.

This also reduces the rcu_exp_handler() function's state space by removing
the direct call that this smp_call_function_single() uses to emulate the
requested self-IPI.

Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
[ paulmck: Use get_cpu() instead of preempt_disable() per Joel Fernandes. ]
---
 kernel/rcu/tree_exp.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 9c990df880d1..5390618787b6 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -384,7 +384,12 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
 			mask_ofl_test |= mask;
 			continue;
 		}
+		if (get_cpu() == cpu) {
+			put_cpu();
+			continue;
+		}
 		ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
+		put_cpu();
 		if (!ret) {
 			mask_ofl_ipi &= ~mask;
 			continue;
-- 
2.17.1