linux-kernel - [RFC PATCH 5/8] entry: Explicitly flush pending rcuog wakeup before last rescheduling points

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Date:   Sat,  9 Jan 2021 03:05:33 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org
Subject: [RFC PATCH 5/8] entry: Explicitly flush pending rcuog wakeup before last rescheduling points

Following the idle loop model, cleanly check for pending rcuog wakeup
before the last rescheduling point on resuming to user mode. This
way we can avoid to do it from rcu_user_enter() with the last resort
self-IPI hack that enforces rescheduling.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar<mingo@...nel.org>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---
 kernel/entry/common.c |  6 ++++++
 kernel/rcu/tree.c     | 12 +++++++-----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 378341642f94..8f3292b5f9b7 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -178,6 +178,9 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
 		/* Architecture specific TIF work */
 		arch_exit_to_user_mode_work(regs, ti_work);
 
+		/* Check if any of the above work has queued a deferred wakeup */
+		rcu_nocb_flush_deferred_wakeup();
+
 		/*
 		 * Disable interrupts and reevaluate the work flags as they
 		 * might have changed while interrupts and preemption was
@@ -197,6 +200,9 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)
 
 	lockdep_assert_irqs_disabled();
 
+	/* Flush pending rcuog wakeup before the last need_resched() check */
+	rcu_nocb_flush_deferred_wakeup();
+
 	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
 		ti_work = exit_to_user_mode_loop(regs, ti_work);
 
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2920dfc9f58c..3c4c0d5cea65 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -705,12 +705,14 @@ noinstr void rcu_user_enter(void)
 
 	lockdep_assert_irqs_disabled();
 	/*
-	 * We may be past the last rescheduling opportunity in the entry code.
-	 * Trigger a self IPI that will fire and reschedule once we resume to
-	 * user/guest mode.
+	 * Other than generic entry implementation, we may be past the last
+	 * rescheduling opportunity in the entry code. Trigger a self IPI
+	 * that will fire and reschedule once we resume in user/guest mode.
 	 */
-	if (do_nocb_deferred_wakeup(rdp) && need_resched())
-		irq_work_queue(this_cpu_ptr(&late_wakeup_work));
+	if (!IS_ENABLED(CONFIG_GENERIC_ENTRY) || (current->flags & PF_VCPU)) {
+		if (do_nocb_deferred_wakeup(rdp) && need_resched())
+			irq_work_queue(this_cpu_ptr(&late_wakeup_work));
+	}
 
 	rcu_eqs_enter(true);
 }
-- 
2.25.1