linux-kernel - Re: gcov: enable GCOV_PROFILE_ALL for x86

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Message-ID: <20090622105609.GA17456@elte.hu>
Date:	Mon, 22 Jun 2009 12:56:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: gcov: enable GCOV_PROFILE_ALL for x86_64

Another thing i was thinking about:

the GCOV code cannot be enabled in distros right now, due to the 
high compiler-generated overhead, and due to the fact that the gcov 
data structures used are single threaded. (which makes a gcov 
enabled kernel very slow on SMP, due to the global cacheline 
bounces)

IMO it would be _much_ better to implement hardware-assisted 
call-graph tracking:

 - Use the BTS (Branch Trace Store) facilities to hardware-sample 
   all branches+calls (optionally, dynamically enable-able)

 - Post-process the raw branch trace information (in the kernel
   BTS-overflow irq handler) to calculate call-coverage information.

Unlike the unconditional GCC based GCOV stuff that is currently 
upstream, BTS tracing is supported by a large range of hardware and 
it can be enabled _transparently_, so it could be built in and 
enabled by distros too, to test code coverage.

Would you be interested in looking at (and implementing) this?

Thanks,

	Ingo
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