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Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS

To: rostedt@goodmis.org
Subject: Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS
From: David Daney <ddaney@caviumnetworks.com>
Date: Wed, 21 Oct 2009 11:25:41 -0700
Cc: Wu Zhangjin <wuzhangjin@gmail.com>, linux-kernel@vger.kernel.org, linux-mips@linux-mips.org, Thomas Gleixner <tglx@linutronix.de>, Ralf Baechle <ralf@linux-mips.org>, Nicholas Mc Guire <der.herr@hofr.at>
In-reply-to: <1256148562.18347.3264.camel@gandalf.stny.rr.com>
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References: <028867b99ec532b84963a35e7d552becc783cafc.1256135456.git.wuzhangjin@gmail.com> <3f0d3515f74a58f4cfd11e61b62a129fdc21e3a7.1256135456.git.wuzhangjin@gmail.com> <ea8aa927fbd184b54941e4c2ae0be8ea0b4f6b8a.1256135456.git.wuzhangjin@gmail.com> <96110ea5dd4d3d54eb97d0bb708a5bd81c7a50b5.1256135456.git.wuzhangjin@gmail.com> <5dda13e8e3a9c9dba4bb7179183941bda502604f.1256135456.git.wuzhangjin@gmail.com> <af3ec1b5cd06b6f6a461c9fa7d09a51fabccb08d.1256135456.git.wuzhangjin@gmail.com> <a6f2959a69b6a77dd32cc36a5c8202f97d524f1e.1256135456.git.wuzhangjin@gmail.com> <53bdfdd95ec4fa00d4cc505bb5972cf21243a14d.1256135456.git.wuzhangjin@gmail.com> <1256141540.18347.3118.camel@gandalf.stny.rr.com> <4ADF38D5.9060100@caviumnetworks.com> <1256143568.18347.3169.camel@gandalf.stny.rr.com> <4ADF3FE0.5090104@caviumnetworks.com> <1256145813.18347.3210.camel@gandalf.stny.rr.com> <4ADF4982.9010306@caviumnetworks.com> <1256148562.18347.3264.camel@gandalf.stny.rr.com>
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Steven Rostedt wrote:
[...]
We're not doing back traces. We need to modify the return of the
function being called. Note, the above functions that end with ";" are
leaf functions. Non leaf functions show "{" and end with "}".

The trick here is to find a reliable way to modify the return address.


Well that's what I get for chiming in without fully understanding the context.

I think it is fine to search backwards from the ($ra - 8) of the call to _mcount until you encounter the adjustment of $sp. If you encounter a store of $ra, you can assume that the function will use the stored value when returning, otherwise modify the value restored to $ra (passed in $at). That is probably what you were doing to begin with.

David Daney

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