linux-kernel - Re: [PATCH 10/17] hyper-v: trace vmbus

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Message-ID: <20171031091028.7fa53a97@gandalf.local.home>
Date:   Tue, 31 Oct 2017 09:10:28 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     olaf@...fle.de, sthemmin@...rosoft.com, jasowang@...hat.com,
        linux-kernel@...r.kernel.org, marcelo.cerri@...onical.com,
        apw@...onical.com, devel@...uxdriverproject.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        leann.ogasawara@...onical.com
Subject: Re: [PATCH 10/17] hyper-v: trace vmbus_open()

On Tue, 31 Oct 2017 13:48:00 +0100
Greg KH <gregkh@...uxfoundation.org> wrote:

> > I don't see how that information can be extracted easily without a
> > tracepoint here. Am I missing something?  
> 
> Wasn't one of the outcomes of the conference last week the fact that for
> ftrace + ebpf we could get access to the structures of the function
> parameters?  Or that work would soon be showing up?

I told Linus that I'll start building an infrastructure on function
tracing to see what we can do. But it may be very limited in features.
I don't believe eBPF can follow arbitrary data structure pointers
without helper functions. Which doesn't exist for this type of code yet.

> 
> It just feels "wrong" to add a tracepoint for a function call, like it
> is a duplication of work/functionality we already have.

We don't already have it. We may have something in a year (or two) that
may be able to get all the data that is requested here. But it's going
to take lots of RFC patch sets and brain storming to come up with
something that everyone is satisfied with.

In other words, the functionality is currently in vaporware state.

-- Steve