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SubjectRe: [PATCH] hrtimer: Interrupt storm on clock_settime
Date
On Thu, Jan 28 2021 at 15:02, Mikael Beckius wrote:

> During clock_settime absolute realtime timers may get updated to expire
> sooner in absolute monotonic time but if hrtimer_force_reprogram is
> called as part of a clock_settime and the next hard hrtimer expires
> before the next soft hrtimer softirq_expires_next will not be updated
> to reflect this change (assuming the realtime timer is a soft timer).
>
> This means that if the next soft hrtimer expires before
> softirq_expires_next but after now no soft hrtimer interrupt will be
> raised in hrtimer_interrupt which will instead retry tick_program_event
> three times before forcing a tick_program_event using a very short delay
> entering hrtimer_interrupt again almost immediately repeating the
> process over and over again until now exceeds softirq_expires_next and a
> soft hrtimer interrupt is finally raised.

Duh.

> This patch aims to solve this by always updating softirq_expires_next if
> a soft hrtimer exists.

git grep 'This patch' Documentation/process/

> Signed-off-by: Mikael Beckius <mikael.beckius@windriver.com>
> ---
> kernel/time/hrtimer.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 743c852e10f2..e4c233f404b1 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -633,7 +633,7 @@ hrtimer_force_reprogram(struct hrtimer_cpu_base *cpu_base, int skip_equal)
> */
> expires_next = __hrtimer_get_next_event(cpu_base, HRTIMER_ACTIVE_ALL);
>
> - if (cpu_base->next_timer && cpu_base->next_timer->is_soft) {
> + if (cpu_base->softirq_next_timer) {

> /*
> * When the softirq is activated, hrtimer has to be
> * programmed with the first hard hrtimer because soft
> @@ -643,7 +643,8 @@ hrtimer_force_reprogram(struct hrtimer_cpu_base *cpu_base, int skip_equal)
> expires_next = __hrtimer_get_next_event(cpu_base,
> HRTIMER_ACTIVE_HARD);
> else
> - cpu_base->softirq_expires_next = expires_next;
> + cpu_base->softirq_expires_next = __hrtimer_get_next_event(cpu_base,
> + HRTIMER_ACTIVE_SOFT);

That works, but we can spare the double scan completely and make the
code understandable. See below.

Thanks,

tglx
---

--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -626,26 +626,25 @@ static inline int hrtimer_hres_active(vo
static void
hrtimer_force_reprogram(struct hrtimer_cpu_base *cpu_base, int skip_equal)
{
- ktime_t expires_next;
+ ktime_t expires_next, soft = KTIME_MAX;

/*
- * Find the current next expiration time.
+ * If soft interrupt has already been activated, ignore the soft
+ * base. It will be handled in the already raised soft interrupt.
*/
- expires_next = __hrtimer_get_next_event(cpu_base, HRTIMER_ACTIVE_ALL);
-
- if (cpu_base->next_timer && cpu_base->next_timer->is_soft) {
+ if (!cpu_base->softirq_activated) {
+ soft = __hrtimer_get_next_event(cpu_base, HRTIMER_ACTIVE_SOFT);
/*
- * When the softirq is activated, hrtimer has to be
- * programmed with the first hard hrtimer because soft
- * timer interrupt could occur too late.
+ * Update the soft expiry time. clock_settime() might have
+ * affected it.
*/
- if (cpu_base->softirq_activated)
- expires_next = __hrtimer_get_next_event(cpu_base,
- HRTIMER_ACTIVE_HARD);
- else
- cpu_base->softirq_expires_next = expires_next;
+ cpu_base->softirq_expires_next = soft;
}

+ expires_next = __hrtimer_get_next_event(cpu_base, HRTIMER_ACTIVE_HARD);
+ if (expires_next > soft)
+ expires_next = soft;
+
if (skip_equal && expires_next == cpu_base->expires_next)
return;

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