slfritchie/otp — pull request #1 by slfritchie (2011-11-11 21:23:44+00:00)
Title: DTrace review (this PR will be ignored by Ericsson)

N.B. This pull request will be completely ignored by Ericsson's OTP team.

Since it's been quite a while since I've written C code, *and* I
haven't done any significant hacking on the VM itself in years, it's
quite likely that I haven't done things in 100% proper style. Or
my co-collaborators Dustin Sallings (CouchBase) or Michal Ptaszek
(Erlang Solutions). My intent for this patch is to start discussion
and review of DTrace support for consideration for the R15 release.

For additional background on the motivation for this work, please
see the slides for the presentation at the Erlang User Conference 2011
in Stockholm:
https://www.erlang-factory.com/upload/presentations/462/euc2011-draft2.pdf

Add autoconf support: use "./configure --enable-dtrace" on all supported
platforms:
* OS X Snow Leopard or later
* Solaris 10 or OpenSolaris
* Linux, via SystemTap's DTrace compatibility packages

See the file `erts/emulator/beam/erlang_dtrace.d` for the definition
of all DTrace probes in the virtual machine so far.

Example D scripts can be found in `lib/dtrace/examples`. Note that if
you see the error message `{name of probe} does not match any probes`,
then there is no Erlang VM process + DTrace probes running. To fix,
start a DTrace-enabled VM or remove `-q` from the `dtrace` command line.

The `lib/dtrace` directory contains a small code-only OTP application
that contains code that allows Erlang code to trigger a DTrace probe.
Dynamic creation & deletion of DTrace probes is not currently
supported, so the `dtrace:p()` function is hacked to allow a variable
number of arguments (up to four integers and up to four strings) to be
used. See the comments at the top of `lib/dtrace/src/dtrace.c` for
more detail.

One feature that may be controversial is the notion I've introduced
of a special process dictionary key that can be used by Erlang code to
tag I/O operations for an application-specific purpose. Right now,
that tag's name is `dtrace_utag`. The dictionary keys used by `sys`
and other modules start with a dollar sign. Perhaps there is some
convention (but not a dollar sign?) that this tag should use?

The purpose of the process dictionary key is to allow the tag to
be included in trace messages, e.g. for file I/O, without changing the
API of the `file.erl` module's functions. For example, here's a use
of the tag when calling the `file:rename/2` function:

(bar@sbb2)1> put(dtrace_utag, "GGOOOAAALL!!!!!").
undefined

(bar@sbb2)2> dtrace:init().
ok

%% Now start both the `user-probe.d` and `efile_drv.d` D scripts
%% found in the `lib/dtrace/examples` directory.

(bar@sbb2)3> dtrace:p(7, 8, 9, "one", "four").
true

%% The output from the `user-probe.d` script:
<0.40.0> GGOOOAAALL!!!!! 7 8 9 0 'one' 'four' '' ''

(bar@sbb2)4> file:rename("old-name", "new-name").
{error,enoent}

%% The output from the `efile_drv.d` script:
async I/O pool port #Port<0.59> queue len 1
async I/O pool port #Port<0.59> queue len 0
efile_drv enter tag={1,110} user tag GGOOOAAALL!!!!! | RENAME (12) | args: old-name new-name , 0 0 (port #Port<0.59>)
async I/O worker tag={1,110} | RENAME (12) | efile_drv-int_entry
async I/O worker tag={1,110} | RENAME (12) | efile_drv-int_return
efile_drv return tag={1,110} user tag GGOOOAAALL!!!!! | RENAME (12) | errno 2

I'm not exactly happy with this choice of tagging, namely using
`put(dtrace_utag, Tag::list())`. But this is an experiment, so
we'll see how it goes. I can't imagine changing the API for
all file.erl functions in order pass the tag explicitly.

Some modules have some extensive (ab)use of the C preprocessor to
reduce the amount of #ifdefs that clutter the code. In several places,
I have not #ifdef'ed automatic variables because of clutter. For the
same reason, there are a handful of cases where I added DTrace-related
members to a struct definition without an #ifdef. I feel that the
result is easier to read than earlier drafts where I did use many more
`https://github.com/slfritchie/otp/tree/dtrace-experiment+michal2` if
you're curious.) I expect there may be some debate about whether the
bloat of the affected structs is worthwhile. I erred on adding stuff
to structs, especially in the efile_drv.c driver, not having a full
grasp on what was thread-safe and what was not ... so I erred on the
side of caution.

The efile_drv.c has a work-around for a crazy GCC optimization bug.
Thank goodness for Google, I dunno how I would've found a work-around
for this silly thing. Many thanks to Trond Norbye for writing clearly
about the problem in a membase Git repo commit message.

/*
* A note on probe naming: if "__" appears in a provider probe
* definition, then two things happen during compilation:
*
* 1. The "__" will turn into a hypen, "-", for the probe name.
* 2. The "__" will turn into a single underscore, "_", for the
* macro names and function definitions that the compiler and
* C developers will see.
*
* We'll try to use the following naming convention. We're a bit
* limited because, as a USDT probe, we can only specify the 4th part
* of the probe name, e.g. erlang*:::mumble. The 2nd part of the
* probe name is always going to be "beam" or "beam.smp", and the 3rd
* part of the probe name will always be the name of the function
* that's calling the probe.
*
* So, all probes will be have names defined in this file using the
* convention category__name or category__sub_category__name. This
* will translate to probe names of category-name or
* category-sub_category-name.
*
* Each of "category", "sub_category", and "name" may have underscores
* but may not have hyphens.
*/

Add tentative support for sequential tracing sending, queueing, and
receiving a message. I don't believe I've fully covered all the major
places where it would be useful to have the sequential trace token info
in a probe -- guidance from the OTP team would be helpful, if there's
time to do that kind of review.

Add global variable `erts_this_node_sysname`.

The purpose of this new global variable is to have quick
access to the local node name without having to acquire
locks to look at the `erts_this_node` variable safely.
[Though, after some later view, this isn't really required??]