linux-kernel - [PATCH v2 18/25] timekeeping: Rework change_clocksource() to use shadow

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Message-Id: <20241009-devel-anna-maria-b4-timers-ptp-timekeeping-v2-18-554456a44a15@linutronix.de>
Date: Wed, 09 Oct 2024 10:29:11 +0200
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: John Stultz <jstultz@...gle.com>, 
 Frederic Weisbecker <frederic@...nel.org>, 
 Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
 Miroslav Lichvar <mlichvar@...hat.com>, 
 Richard Cochran <richardcochran@...il.com>, 
 Christopher S Hall <christopher.s.hall@...el.com>, 
 Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: [PATCH v2 18/25] timekeeping: Rework change_clocksource() to use
 shadow_timekeeper

From: Anna-Maria Behnsen <anna-maria@...utronix.de>

Updates of the timekeeper can be done by operating on the shadow timekeeper
and afterwards copying the result into the real timekeeper. This has the
advantage, that the sequence count write protected region is kept as small
as possible.

Convert change_clocksource() to use this scheme.

Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
---
 kernel/time/timekeeping.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 051041e92e54..6d21e9bf5c35 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1596,9 +1596,7 @@ static void __timekeeping_set_tai_offset(struct timekeeper *tk, s32 tai_offset)
  */
 static int change_clocksource(void *data)
 {
-	struct timekeeper *tk = &tk_core.timekeeper;
 	struct clocksource *new = data, *old = NULL;
-	unsigned long flags;
 
 	/*
 	 * If the clocksource is in a module, get a module reference.
@@ -1614,16 +1612,14 @@ static int change_clocksource(void *data)
 		return 0;
 	}
 
-	raw_spin_lock_irqsave(&tk_core.lock, flags);
-	write_seqcount_begin(&tk_core.seq);
-
-	timekeeping_forward_now(tk);
-	old = tk->tkr_mono.clock;
-	tk_setup_internals(tk, new);
-	timekeeping_update(&tk_core, tk, TK_UPDATE_ALL | TK_MIRROR);
+	scoped_guard (raw_spinlock_irqsave, &tk_core.lock) {
+		struct timekeeper *tk = &tk_core.shadow_timekeeper;
 
-	write_seqcount_end(&tk_core.seq);
-	raw_spin_unlock_irqrestore(&tk_core.lock, flags);
+		timekeeping_forward_now(tk);
+		old = tk->tkr_mono.clock;
+		tk_setup_internals(tk, new);
+		timekeeping_update_staged(&tk_core, TK_UPDATE_ALL);
+	}
 
 	if (old) {
 		if (old->disable)

-- 
2.39.5