linux-kernel - Re: [PATCH 0/3] ARM Coresight: Enhance ETM tracing control

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Date:	Wed, 4 Dec 2013 20:01:04 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Adrien Vergé <adrienverge@...il.com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	Will Deacon <will.deacon@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH 0/3] ARM Coresight: Enhance ETM tracing control

On Wed, Dec 04, 2013 at 10:49:25PM -0500, Adrien Vergé wrote:
> 2013/12/4 Greg Kroah-Hartman <gregkh@...uxfoundation.org>:
> > How much overhead does the existing tracing code have on ARM?  Is ETM
> > still even needed?  Why not just use ETM for the core tracing code
> > instead?
> 
> Coresight ETM is not just faster than /sys/kernel/debug/tracing, it
> provides more detailed and customisable info. For instance, you can
> trace every load, store, instruction fetch, along with the number of
> cycles taken, with almost zero-overhead.

Can't you already do that with the 'perf' tool the kernel provides
without the ETM driver?

> > What's wrong with the in-kernel tracing logic that you can't use that
> > instead of the ETM stuff?
> 
> ETM has a different purpose. Integrating it in
> /sys/kernel/debug/tracing would not take advantage of all its
> features.

What is it's purpose then?  At first glance, this seems to be exactly
what 'perf' provides already.  Doesn't perf work on ARM today?

thanks,

greg k-h
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