Subject: [PATCH] tick/common: Align tick period with the HZ tick.
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Tue, 18 Apr 2023 14:26:39 +0200

With HIGHRES enabled tick_sched_timer() is programmed every jiffy to
expire the timer_list timers. This timer is programmed accurate in
respect to CLOCK_MONOTONIC so that 0 seconds and nanoseconds is the
first tick and the next one is 1000/CONFIG_HZ ms later. For HZ=250 it is
every 4 ms and so based on the current time the next tick can be
computed.

This accuracy broke since the commit mentioned below because the jiffy
based clocksource is initialized with higher accuracy in
read_persistent_wall_and_boot_offset(). This higher accuracy is
inherited during the setup in tick_setup_device(). The timer still fires
every 4ms with HZ=250 but timer is no longer aligned with
CLOCK_MONOTONIC with 0 as it origin but has an offset in the us/ns part
of the timestamp. The offset differs with every boot and makes it
impossible for user land to align with the tick.

Align the tick period with CLOCK_MONOTONIC ensuring that it is always a
multiple of 1000/CONFIG_HZ ms.

Fixes: 857baa87b6422 ("sched/clock: Enable sched clock early")
Reported-by: Gusenleitner Klaus <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Signed-off-by: Thomas Gleixner <[email-masked]>
Link: https://lore.kernel.org/[email-masked]
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/time/tick-common.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

Index: linux-6.3.0-rt11/kernel/time/tick-common.c
===================================================================
--- linux-6.3.0-rt11.orig/kernel/time/tick-common.c
+++ linux-6.3.0-rt11/kernel/time/tick-common.c
@@ -218,9 +218,19 @@ static void tick_setup_device(struct tic
 		 * this cpu:
 		 */
 		if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
+			ktime_t next_p;
+			u32 rem;
+
 			tick_do_timer_cpu = cpu;
 
-			tick_next_period = ktime_get();
+			next_p = ktime_get();
+			div_u64_rem(next_p, TICK_NSEC, &rem);
+			if (rem) {
+				next_p -= rem;
+				next_p += TICK_NSEC;
+			}
+
+			tick_next_period = next_p;
 #ifdef CONFIG_NO_HZ_FULL
 			/*
 			 * The boot CPU may be nohz_full, in which case set