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From:   Anna-Maria Behnsen <anna-maria@linutronix.de>
To:     linux-kernel@vger.kernel.org
Cc:     Peter Zijlstra <peterz@infradead.org>,
        John Stultz <jstultz@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Eric Dumazet <edumazet@google.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Arjan van de Ven <arjan@infradead.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Rik van Riel <riel@surriel.com>,
        Anna-Maria Behnsen <anna-maria@linutronix.de>,
        Frederic Weisbecker <frederic@kernel.org>
Subject: [PATCH v4 04/16] timer: Rework idle logic
Date:   Fri,  4 Nov 2022 15:57:25 +0100
Message-Id: <20221104145737.71236-5-anna-maria@linutronix.de>
In-Reply-To: <20221104145737.71236-1-anna-maria@linutronix.de>
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From: Thomas Gleixner <tglx@linutronix.de>

To improve readability of the code, split base->idle calculation and
expires calculation into separate parts.

Thereby the following subtle change happens if the next event is just one
jiffy ahead and the tick was already stopped: Originally base->is_idle
remains true in this situation. Now base->is_idle turns to false. This may
spare an IPI if a timer is enqueued remotely to an idle CPU that is going
to tick on the next jiffy.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
---
v4: Change condition to force 0 delta and update commit message (Frederic)
---
 kernel/time/timer.c | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 4547d2efcb86..0e9f609f5a38 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1727,21 +1727,20 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 			base->clk = nextevt;
 	}
 
-	if (time_before_eq(nextevt, basej)) {
-		expires = basem;
-		base->is_idle = false;
-	} else {
-		if (base->timers_pending)
-			expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
-		/*
-		 * If we expect to sleep more than a tick, mark the base idle.
-		 * Also the tick is stopped so any added timer must forward
-		 * the base clk itself to keep granularity small. This idle
-		 * logic is only maintained for the BASE_STD base, deferrable
-		 * timers may still see large granularity skew (by design).
-		 */
-		if ((expires - basem) > TICK_NSEC)
-			base->is_idle = true;
+	/*
+	 * Base is idle if the next event is more than a tick away. Also
+	 * the tick is stopped so any added timer must forward the base clk
+	 * itself to keep granularity small. This idle logic is only
+	 * maintained for the BASE_STD base, deferrable timers may still
+	 * see large granularity skew (by design).
+	 */
+	base->is_idle = time_after(nextevt, basej + 1);
+
+	if (base->timers_pending) {
+		/* If we missed a tick already, force 0 delta */
+		if (time_before(nextevt, basej))
+			nextevt = basej;
+		expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
 	}
 	raw_spin_unlock(&base->lock);
 
-- 
2.30.2