* Query regarding __hrtimer_get_next_event()
@ 2017-10-26 9:04 Neeraj Upadhyay
2017-10-26 12:57 ` Thomas Gleixner
0 siblings, 1 reply; 4+ messages in thread
From: Neeraj Upadhyay @ 2017-10-26 9:04 UTC (permalink / raw)
To: tglx; +Cc: sramana, sboyd, linux-kernel
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
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Query regarding __hrtimer_get_next_event()
2017-10-26 9:04 Query regarding __hrtimer_get_next_event() Neeraj Upadhyay
@ 2017-10-26 12:57 ` Thomas Gleixner
2017-10-26 14:23 ` Neeraj Upadhyay
0 siblings, 1 reply; 4+ messages in thread
From: Thomas Gleixner @ 2017-10-26 12:57 UTC (permalink / raw)
To: Neeraj Upadhyay; +Cc: sramana, sboyd, LKML, John Stultz
On Thu, 26 Oct 2017, Neeraj Upadhyay wrote:
> 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.
>
>
> 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?
Can you please explain how you managed to have a negative base->offset?
Thanks,
tglx
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Query regarding __hrtimer_get_next_event()
2017-10-26 12:57 ` Thomas Gleixner
@ 2017-10-26 14:23 ` Neeraj Upadhyay
2017-10-27 7:32 ` Thomas Gleixner
0 siblings, 1 reply; 4+ messages in thread
From: Neeraj Upadhyay @ 2017-10-26 14:23 UTC (permalink / raw)
To: Thomas Gleixner; +Cc: sramana, sboyd, LKML, John Stultz
Hi tglx,
Forgot to mention, we are using kernel stable version 3.18
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/kernel/time/timekeeping.c?h=v3.18.77
wall time is being set to a value close to epoch
but less than epoch + current uptime.
Looks like
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/kernel/time?id=e1d7ba8735551ed79c7a0463a042353574b96da3
handles this case, but is not present in 3.18 tree.
We will try by pulling this patch to 3.18 tree.
int do_settimeofday(const struct timespec *tv)
{
xt = tk_xtime(tk);
ts_delta.tv_sec = tv->tv_sec - xt.tv_sec;
ts_delta.tv_nsec = tv->tv_nsec - xt.tv_nsec;
<snip>
tk_set_wall_to_mono(tk, timespec64_sub(tk->wall_to_monotonic, ts_delta));
}
xt = (
tv_sec = 1508807131,
tv_nsec = 223767601)
2017/10/24 9:5:31
tk->wall_to_monotonic = (
tv_sec = -1508803094,
tv_nsec = 15152092)
ts_delta = (tv_sec = -1508803200, tv_nsec = -767601)
wall_to_monotonic is bigger than the ts_delta, so leading
to wall_to_monotonic become positive value, resulting in
-ve off_real.
Thanks
Neeraj
On 10/26/2017 06:27 PM, Thomas Gleixner wrote:
> On Thu, 26 Oct 2017, Neeraj Upadhyay wrote:
>> 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.
>>
>>
>> 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?
> Can you please explain how you managed to have a negative base->offset?
>
> Thanks,
>
> tglx
--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Query regarding __hrtimer_get_next_event()
2017-10-26 14:23 ` Neeraj Upadhyay
@ 2017-10-27 7:32 ` Thomas Gleixner
0 siblings, 0 replies; 4+ messages in thread
From: Thomas Gleixner @ 2017-10-27 7:32 UTC (permalink / raw)
To: Neeraj Upadhyay; +Cc: sramana, sboyd, LKML, John Stultz
On Thu, 26 Oct 2017, Neeraj Upadhyay wrote:
> Hi tglx,
>
> Forgot to mention, we are using kernel stable version 3.18
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/tree/kernel/time/timekeeping.c?h=v3.18.77
>
> wall time is being set to a value close to epoch
> but less than epoch + current uptime.
>
> Looks like
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/kernel/time?id=e1d7ba8735551ed79c7a0463a042353574b96da3
>
> handles this case, but is not present in 3.18 tree.
> We will try by pulling this patch to 3.18 tree.
Sigh. This should have been tagged for stable.
Thanks,
tglx
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2017-10-27 7:32 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2017-10-26 9:04 Query regarding __hrtimer_get_next_event() Neeraj Upadhyay
2017-10-26 12:57 ` Thomas Gleixner
2017-10-26 14:23 ` Neeraj Upadhyay
2017-10-27 7:32 ` Thomas Gleixner
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).