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

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Date:	Mon, 25 Feb 2013 14:38:18 +0800
From:	Jason Liu <liu.h.jason@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: too many timer retries happen when do local timer swtich with
 broadcast timer

Thomas,

2013/2/23 Thomas Gleixner <tglx@...utronix.de>:
> 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.
>> > >
>> > > 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.

I have applied your patch and tested, there is NO warning at all then.
I think your theory is correct.

>
> 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 prefer the solution 2).

>
> Thanks,
>
>         tglx
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