Re: too many timer retries happen when do local timer swtich with broadcast timer

Santosh Shilimkar
2013-02-25T06:11:41Z

On Saturday 23 February 2013 12:22 AM, Thomas Gleixner wrote:
> On Fri, 22 Feb 2013, Lorenzo Pieralisi wrote:
>> On Fri, Feb 22, 2013 at 03:03:02PM +0000, Thomas Gleixner wrote:
>>> On Fri, 22 Feb 2013, Lorenzo Pieralisi wrote:
>>>> On Fri, Feb 22, 2013 at 12:07:30PM +0000, Thomas Gleixner wrote:
>>>>> Now we could make use of that and avoid going deep idle just to come
>>>>> back right away via the IPI. Unfortunately the notification thingy has
>>>>> no return value, but we can fix that.
>>>>>
>>>>> To confirm that theory, could you please try the hack below and add
>>>>> some instrumentation (trace_printk)?
>>>>
>>>> Applied, and it looks like that's exactly why the warning triggers, at least
>>>> on the platform I am testing on which is a dual-cluster ARM testchip.
>>>>
I too confirm that the warnings cause is same.

>>>> There is a still time window though where the CPU (the IPI target) can get
>>>> back to idle (tick_broadcast_pending still not set) before the CPU target of
>>>> the broadcast has a chance to run tick_handle_oneshot_broadcast (and set
>>>> tick_broadcast_pending), or am I missing something ?
>>>
>>> Well, the tick_broadcast_pending bit is uninteresting if the
>>> force_broadcast bit is set. Because if that bit is set we know for
>>> sure, that we got woken with the cpu which gets the broadcast timer
>>> and raced back to idle before the broadcast handler managed to
>>> send the IPI.
>>
>> Gah, my bad sorry, I mixed things up. I thought
>>
>> tick_check_broadcast_pending()
>>
>> was checking against the tick_broadcast_pending mask not
>>
>> tick_force_broadcast_mask
>
> Yep, that's a misnomer. I just wanted to make sure that my theory is
> correct. I need to think about the real solution some more.
>
> We have two alternatives:
>
> 1) Make the clockevents_notify function have a return value.
>
> 2) Add something like the hack I gave you with a proper name.
>
> The latter has the beauty, that we just need to modify the platform
> independent idle code instead of going down to every callsite of the
> clockevents_notify thing.
>
I agree that 2 is better alternative to avoid multiple changes.
Whichever alternative you choose, will be happy to test it :)

Regards,
Santosh