Query regarding __hrtimer_get_next_event()

[Neeraj Upadhyay]

Hi,

We have one query regarding the __hrtimer_get_next_event().
The expires_next.tv64 is set to 0 if it is < 0. We observed
an hrtimer interrupt storm for one of the hrtimers with
below properties:

* Expires for the hrtimer was set to KTIME_MAX.
* cpu base was HRTIMER_BASE_REALTIME with negative base->offset.
* Due to below sub, expires overflowed to a negative value and
  expires_next.tv64 was set to 0
    expires = ktime_sub(hrtimer_get_expires(timer), base->offset);
* Due to this, clockevent was programmed to min_delta_ns, everytime
  as __hrtimer_get_next_event() returned 0.


  static ktime_t __hrtimer_get_next_event(...)
  {
    <snip>
    for (; active; base++, active >>= 1) {

      <snip>
      timer = container_of(next, struct hrtimer, node);
      expires = ktime_sub(hrtimer_get_expires(timer),
        base->offset);
      if (expires.tv64 < expires_next.tv64) {
        expires_next = expires;
        hrtimer_update_next_timer(cpu_base, timer);
      }
    }
    <snip>
    if (expires_next.tv64 < 0)
      expires_next.tv64 = 0;
    return expires_next;
  }

This may not be a valid use case (queuing a hrtimer with KTIME_MAX)
expires, but should we guard the hrtimer next event code against
this by using KTIME_MAX upper bound. Is something like below a
proper way to guard it? Or am I missing something here?

expires = ktime_sub(hrtimer_get_expires(timer), base->offset);
+  /*
+   * if expires is a very high positive value and base->offset is
+   * negative, expires can overflow and get negative value. Set
+   * expires to KTIME_MAX, if we encounter this.
+   */
+   if (hrtimer_get_expires(timer).tv64 > 0 &&
+      base->offset.tv64 < 0 && expires.tv64 < 0)
+       expires.tv64 = KTIME_MAX;
+


Thanks
Neeraj

--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation