linux-ext4 - Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing

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Date:   Mon, 28 Nov 2016 09:33:27 +0100
From:   Jan Kara <jack@...e.cz>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dave Chinner <david@...morbit.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>
Subject: Re: [PATCH 3/6] dax: add tracepoint infrastructure, PMD tracing

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@...e.com>
SUSE Labs, CR
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