From: Jan Kara <jack@suse.cz>
Subject: Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing
Date: Mon, 28 Nov 2016 09:33:27 +0100
Message-ID: <20161128083327.GA2590@quack2.suse.cz>
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Dave Chinner <david@fromorbit.com>,
	Ross Zwisler <ross.zwisler@linux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>,
	Dan Williams <dan.j.williams@intel.com>,
	Ingo Molnar <mingo@redhat.com>, Jan Kara <jack@suse.cz>,
	Matthew Wilcox <mawilcox@microsoft.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	"linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	"linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>
To: Theodore Ts'o <tytso@mit.edu>
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On Fri 25-11-16 16:48:40, Ted Tso wrote:
> On Fri, Nov 25, 2016 at 11:51:26AM -0800, Linus Torvalds wrote:
> > We do have filesystem code that is just disgusting. As an example:
> > fs/afs/ tends to have these crazy "_enter()/_exit()" macros in every
> > single function. If you want that, use the function tracer. That seems
> > to be just debugging code that has been left around for others to
> > stumble over. I do *not* believe that we should encourage that kind of
> > "machine gun spray" use of tracepoints.
> 
> There is a reason why people want to be able to do that, and that's
> because kprobes doesn't give you access to the arguments and return
> codes to the functions.  Maybe there could be a way to do this more
> easily using DWARF information and EBPF magic, perhaps?  It won't help
> for inlined functions, of course, but most of the functions where
> people want to do this aren't generally functions which are going to
> be inlined, but rather things like write_begin, writepages, which are
> called via a struct ops table and so will never be inlined to begin
> with.

Actually, you can print register & stack contents from a kprobe and you can
get a function return value from a kretprobe (see
Documentation/trace/kprobetrace.txt). Since calling convention is fixed
(arg 1 in RDI, arg 2 in RSI...) you can relatively easily dump function
arguments on entry and dump return value on return for arbitrary function
of your choice. I was already debugging issues like that several times (in
VFS actually because of missing trace points ;)). You can even create a
kprobe to dump register contents in the middle of the function (although
there it takes more effort reading the dissasembly to see what you are
interested in).

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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