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

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Message-ID: <alpine.LFD.2.02.1302221946070.22263@ionos>
Date:	Fri, 22 Feb 2013 19:52:14 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
cc:	Santosh Shilimkar <santosh.shilimkar@...com>,
	Jason Liu <liu.h.jason@...il.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

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.

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.

Thanks,

	tglx
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