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

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Message-ID: <CANsc=4UGi5sMdUW5qQDzEJx4h8NDU+-b=xOLBAZtzbV5ugz3Fg@mail.gmail.com>
Date:	Wed, 4 Dec 2013 22:49:25 -0500
From:	Adrien Vergé <adrienverge@...il.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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

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.

> 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.
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