[tip: timers/core] timers: Fix nextevt calculation when no timers are pending

tip-bot2 for Anna-Maria Behnsen
2023-12-20T15:59:07Z

The following commit has been merged into the timers/core branch of tip:

Commit-ID: da65f29dada7f7cbbf0d6375b88a0316f5f7d6f5
Gitweb: https://git.kernel.org/tip/da65f29dada7f7cbbf0d6375b88a0316f5f7d6f5
Author: Anna-Maria Behnsen <anna-maria@linutronix.de>
AuthorDate: Fri, 01 Dec 2023 10:26:34 +01:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 20 Dec 2023 16:49:39 +01:00

timers: Fix nextevt calculation when no timers are pending

When no timer is queued into an empty timer base, the next_expiry will not
be updated. It was originally calculated as

base->clk + NEXT_TIMER_MAX_DELTA

When the timer base stays empty long enough (> NEXT_TIMER_MAX_DELTA), the
next_expiry value of the empty base suggests that there is a timer pending
soon. This might be more a kind of a theoretical problem, but the fix
doesn't hurt.

Use only base->next_expiry value as nextevt when timers are
pending. Otherwise nextevt will be jiffies + NEXT_TIMER_MAX_DELTA. As all
information is in place, update base->next_expiry value of the empty timer
base as well.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20231201092654.34614-13-anna-maria@linutronix.de

---
kernel/time/timer.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index cf51655..352b161 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1922,8 +1922,8 @@ static u64 cmp_next_hrtimer_event(u64 basem, u64 expires)
u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
{
struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
+ unsigned long nextevt = basej + NEXT_TIMER_MAX_DELTA;
u64 expires = KTIME_MAX;
- unsigned long nextevt;
bool was_idle;

/*
@@ -1936,7 +1936,6 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
raw_spin_lock(&base->lock);
if (base->next_expiry_recalc)
next_expiry_recalc(base);
- nextevt = base->next_expiry;

/*
* We have a fresh next event. Check whether we can forward the
@@ -1945,10 +1944,20 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
__forward_timer_base(base, basej);

if (base->timers_pending) {
+ nextevt = base->next_expiry;
+
/* If we missed a tick already, force 0 delta */
if (time_before(nextevt, basej))
nextevt = basej;
expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
+ } else {
+ /*
+ * Move next_expiry for the empty base into the future to
+ * prevent a unnecessary raise of the timer softirq when the
+ * next_expiry value will be reached even if there is no timer
+ * pending.
+ */
+ base->next_expiry = nextevt;
}

/*