Re: [PATCH] hrtimer: Use hard expiry when updating timers on the same base
flat view
From: Peter Zijlstra <peterz@infradead.org>
Date: 2026-09-08 11:09:52
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lkml
Subsystem:
high-resolution timers, timer wheel, clockevents, the rest · Maintainers:
Anna-Maria Behnsen, Frederic Weisbecker, Thomas Gleixner, Linus Torvalds
On Mon, Sep 07, 2026 at 11:11:33PM +0200, Andrea Parri wrote:
Rearming a queued timer with nonzero slack can leave the timerqueue out
of order. remove_and_enqueue_same_base() checks the new soft expiry
against its neighbours' hard expiries, then stores the new hard expiry
in the node without requeueing it.
For example, with A at 10 and B at 20, rearming A at 11 with slack 30
passes the neighbour check but leaves A's hard expiry of 41 before B's
20. The same function also caches the soft expiry in base->expires_next
when updating or inserting the first timer, giving next-event selection
an earlier deadline than the queue head's hard expiry.
Compute the hard expiry with the same saturating addition used by
hrtimer_set_expires_range_ns(). Use it for the in-place ordering check
and both updates to base->expires_next.
Fixes: eddffab8282e3 ("hrtimer: Keep track of first expiring timer per clock base")
Fixes: 343f2f4dc5425 ("hrtimer: Try to modify timers in place")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>Urgh!
quoted hunk ↗ jump to hunk
--- kernel/time/hrtimer.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-)diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c index 530d61257b9a0..c5940323746db 100644 --- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c@@ -1261,6 +1261,7 @@ static inline bool remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *base, const enum hrtimer_mode mode, ktime_t expires, u64 delta_ns) { + ktime_t hard_expires = ktime_add_safe(expires, ns_to_ktime(delta_ns)); bool was_first = false; /* Remove it from the timer queue if active */@@ -1268,11 +1269,11 @@ remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *b was_first = !timerqueue_linked_prev(&timer->node); /* Try to update in place to avoid the de/enqueue dance */ - if (hrtimer_can_update_in_place(timer, base, expires)) { + if (hrtimer_can_update_in_place(timer, base, hard_expires)) { hrtimer_set_expires_range_ns(timer, expires, delta_ns); trace_hrtimer_start(timer, mode, true); if (was_first) - base->expires_next = expires; + base->expires_next = hard_expires; return was_first; }@@ -1291,7 +1292,7 @@ remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *b /* If it's the first expiring timer now or again, update base */ if (timerqueue_linked_add(&base->active, &timer->node)) { - base->expires_next = expires; + base->expires_next = hard_expires; return true; }
Can't we do something like so?
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 530d61257b9a..8c157c4fa699 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c@@ -1263,13 +1263,16 @@ remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *b { bool was_first = false; + /* Set the new expiry time */ + hrtimer_set_expires_range_ns(timer, expires, delta_ns); + expires = timer->node.expires; + /* Remove it from the timer queue if active */ if (timer->is_queued) { was_first = !timerqueue_linked_prev(&timer->node); /* Try to update in place to avoid the de/enqueue dance */ if (hrtimer_can_update_in_place(timer, base, expires)) { - hrtimer_set_expires_range_ns(timer, expires, delta_ns); trace_hrtimer_start(timer, mode, true); if (was_first) base->expires_next = expires;
@@ -1280,9 +1283,6 @@ remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *b timerqueue_linked_del(&base->active, &timer->node); } - /* Set the new expiry time */ - hrtimer_set_expires_range_ns(timer, expires, delta_ns); - debug_activate(timer, mode, timer->is_queued); base->cpu_base->active_bases |= 1 << base->index;