[PATCH v9 12/32] timers: Fix nextevt calculation when no timers are pending

Anna-Maria Behnsen
2023-12-01T09:27:52Z

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>
---
v9: New patch
---
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 0826018d9873..4dffe966424c 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;

/*
* Pretend that there is no timer pending if the cpu is offline.
@@ -1935,7 +1935,6 @@ 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
@@ -1944,10 +1943,20 @@ 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;
}

/*
--
2.39.2