linux-kernel - Re: oprofile and ARM A9 hardware counter

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Message-ID: <20120220094414.GA13115@mudshark.cambridge.arm.com>
Date:	Mon, 20 Feb 2012 09:44:14 +0000
From:	Will Deacon <will.deacon@....com>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"eranian@...il.com" <eranian@...il.com>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	David Long <dave.long@...aro.org>,
	"b-cousson@...com" <b-cousson@...com>,
	"mans@...sr.com" <mans@...sr.com>,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: oprofile and ARM A9 hardware counter

On Mon, Feb 20, 2012 at 03:19:02AM +0000, Ming Lei wrote:
> On Fri, Feb 17, 2012 at 6:26 PM, Will Deacon <will.deacon@....com> wrote:
> > Not sure I follow. If the frequency is 10000, then we write 0xffffd8f0 to
> > the counter. That means we have a 0xffffd8f0 event window to read the
> 
> The frequency I described is the 'freq' from '-F freq'. On OMAP4, when
> the 'freq'
> is 10000 and the interval for one samle is 100us, the observed counter
> ('left' variable in armpmu_event_set_period) is about 90000, so the written
> value to the hw counter is 0xFFFEA06F, looks the window is wide enough,
> and we may not consider the issue in a737823d for sample-based profiling.

My mistake - I thought you were referring to a period of 10000, but similar
logic follows.

> > There's a trade off between allowing the counter to wrap back around past
> > its previous value or being able to handle overflow on a non-interrupt path.
> 
> I agree, it is not easy to read overflow flag and counter accurately
> from hardware
> directly on a non-interrupt path.
> 
> So do you need me to prepare a new patch, or you will do it by yourself?

If the last one I posted is alright to you (seems that it is), then I'll
update it with the removal of the overflow flag and post it to the list with
you on CC.

I also need to get to the bottom of those warnings.

Thanks,

Will
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