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

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Date:	Fri, 22 Feb 2013 16:03:02 +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 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.

If we did not get woken before the broadcast IPI then the pending bit
is set when we exit the broadcast mode.

> It is a corner case, granted. Best thing would be to check pending IRQs in the
> idle driver back-end (or have always-on local timers :-)).

The latter is definitely the only sane solution.

Thanks,

	tglx
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