Re: [PATCH 1/2] hrtimer: reprogram event for expires=KTIME_MAX in hrtimer_force_reprogram()

[Viresh Kumar]

On 10 May 2014 21:47, Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx> wrote:
> On 05/09/2014 04:27 PM, Viresh Kumar wrote:
>> On 9 May 2014 16:04, Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx> wrote:

>> Ideally, the device should have stopped events as we programmed it in
>> ONESHOT mode. And should have waited for kernel to set it again..
>>
>> But probably that device doesn't have a ONESHOT mode and is firing
>> again and again. Anyway the real problem I was trying to solve wasn't
>> infinite interrupts coming from event dev, but the first extra event that
>> we should have got rid of .. It just happened that we got more problems
>> on this particular board.
>
> So on a timer interrupt the tick device, irrespective of if it is in
> ONESHOT mode or not, is in an expired state. Thus it will continue to
> fire. What has ONESHOT mode got to do with this?

So, the arch specific timer handler must be clearing it I suppose and it
shouldn't have fired again after 5 ms as it is not reprogrammed.

Probably that's an implementation specific stuff.. I have seen timers which
have two modes, periodic: they fire continuously and oneshot: they get
disabled after firing and have to be reprogrammed.

>>> The reason this got exposed in NOHZ_FULL config is because in a normal
>>> NOHZ scenario when the cpu goes idle, and there are no pending timers in
>>> timer_list, even then tick_sched_timer gets cancelled. Precisely the
>>> scenario that you have described.
>>
>> I haven't tried but it looks like this problem will exist there as well.. Who is
>> disabling the event device in that case when tick_sched timer goes off ?
>> The same question that is applicable in this case as well..
>>
>>>    But we don't get continuous interrupts then because the first time we
>>> get an interrupt, we queue the tick_sched_timer and program the tick
>>> device to the time of its expiry and therefore *push* the time at which
>>> your tick device should fire further.
>>
>> Probably not.. We don't get continuous interrupts because that's a special
>> case for my platform. But I am quite sure you would be getting one extra
>> interrupt after tick period, but because we didn't had anything to service
>
> Hmm? I didn't get this. Why would we?  We ensure that if there are no
> pending timers in timer_list the tick_sched_timer is cancelled. We
> cannot get spurious interrupts when there are no pending timers in NOHZ
> mode.

Okay, there are no pending timers to fire and even we have disabled
tick_sched_timer as well.. But the event dev isn't SHUTDOWN or reprogrammed.
And so it must fire after tick interval? Exactly the same issue we are getting
here in NO_HZ_FULL..

And the worst part is we aren't getting these interrupts in traces as well.
Somebody probably need to revisit the trace_irq_handler_entry part as well
to catch such problems.

> Hmm yeah looking at the problem that you are trying to solve, that being
> completely disabling timer interrupts on cpus that are running just one
> process, it appears to me that setting the tick device in SHUTDOWN mode
> is the only way to do so. And you are right. We use SHUTDOWN mode to
> imply that the device can be switched off. Its upto the arch to react to
> it appropriately.

So, from the mail where tglx blasted me off, we have a better solution to
implement now :)

> My concern is on powerpc today when we set the device to SHUTDOWN mode
> we set the decrementer to a MAX value. Which means we will get
> interrupts only spaced out more widely in time. But on NOHZ_FULL mode if
> you are looking at completely disabling tick_sched_timer as long as a
> single process runs then we might need to change the semantics here.

Lets see if we can do some nice stuff with ONESHOT_STOPPED state..
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