Re: [PATCH v2] docs: Use make invocation's -j argument for parallelism

[Jani Nikula]

On Mon, 23 Sep 2019, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Sun, Sep 22, 2019 at 02:03:31PM -0600, Jonathan Corbet wrote:
>> On Thu, 19 Sep 2019 14:44:37 -0700
>> Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> 
>> > While sphinx 1.7 and later supports "-jauto" for parallelism, this
>> > effectively ignores the "-j" flag used in the "make" invocation, which
>> > may cause confusion for build systems. Instead, extract the available
>> 
>> What sort of confusion might we expect?  Or, to channel akpm, "what are the
>> user-visible effects of this bug"?
>
> When I run "make htmldocs -j16" with a pre-1.7 sphinx, it is not
> parallelized. When I run "make htmldocs -j8" with 1.7+ sphinx, it uses
> all my CPUs instead of 8. :)

To be honest, part of the solution should be to require Sphinx 1.8 or
later. Even Debian stable has it. If your distro doesn't have it
(really?), using the latest Sphinx in a virtual environment should be a
matter of:

$ python3 -m venv .venv
$ . .venv/bin/activate
(.venv) $ pip install sphinx sphinx_rtd_theme
(.venv) $ make htmldocs

BR,
Jani.


>
>> > +	-j $(shell python3 $(srctree)/scripts/jobserver-count $(SPHINX_PARALLEL)) \
>> 
>> This (and the shebang line in the script itself) will cause the docs build
>> to fail on systems lacking Python 3.  While we have talked about requiring
>> Python 3 for the docs build, we have not actually taken that step yet.  We
>> probably shouldn't sneak it in here.  I don't see anything in the script
>> that should require a specific Python version, so I think it should be
>> tweaked to be version-independent and just invoke "python".
>
> Ah, no problem. I can fix this. In a quick scan it looked like sphinx
> was python3, but I see now that's just my install. :)
>
>> >  	-b $2 \
>> >  	-c $(abspath $(srctree)/$(src)) \
>> >  	-d $(abspath $(BUILDDIR)/.doctrees/$3) \
>> > diff --git a/scripts/jobserver-count b/scripts/jobserver-count
>> > new file mode 100755
>> > index 000000000000..ff6ebe6b0194
>> > --- /dev/null
>> > +++ b/scripts/jobserver-count
>> > @@ -0,0 +1,53 @@
>> > +#!/usr/bin/env python3
>> > +# SPDX-License-Identifier: GPL-2.0-or-later
>> 
>> By license-rules.rst, this should be GPL-2.0+
>
> Whoops, thanks.
>
>> > +# Extract and prepare jobserver file descriptors from envirnoment.
>> > +try:
>> > +	# Fetch the make environment options.
>> > +	flags = os.environ['MAKEFLAGS']
>> > +
>> > +	# Look for "--jobserver=R,W"
>> > +	opts = [x for x in flags.split(" ") if x.startswith("--jobserver")]
>> > +
>> > +	# Parse out R,W file descriptor numbers and set them nonblocking.
>> > +	fds = opts[0].split("=", 1)[1]
>> > +	reader, writer = [nonblock(int(x)) for x in fds.split(",", 1)]
>> > +except:
>> 
>> So I have come to really dislike bare "except" clauses; I've seen them hide
>> too many bugs.  In this case, perhaps it's justified, but still ... it bugs
>> me ...
>
> Fair enough. I will adjust this (and the later instance).
>
>> 
>> > +	# Any failures here should result in just using the default
>> > +	# specified parallelism.
>> > +	print(default)
>> > +	sys.exit(0)
>> > +
>> > +# Read out as many jobserver slots as possible.
>> > +jobs = b""
>> > +while True:
>> > +	try:
>> > +		slot = os.read(reader, 1)
>> > +		jobs += slot
>> > +	except:
>> 
>> This one, I think, should be explicit; anything other than EWOULDBLOCK
>> indicates a real problem, right?
>> 
>> > +		break
>> > +# Return all the reserved slots.
>> > +os.write(writer, jobs)
>> 
>> You made writer nonblocking, so it seems plausible that we could leak some
>> slots here, no?  Does writer really need to be nonblocking?
>
> Good point. I will fix this too.
>
>> 
>> > +# If the jobserver was (impossibly) full or communication failed, use default.
>> > +if len(jobs) < 1:
>> > +	print(default)
>> > +
>> > +# Report available slots (with a bump for our caller's reserveration).
>> > +print(len(jobs) + 1)
>> 
>> The last question I have is...why is it that we have to do this complex
>> dance rather than just passing the "-j" option through directly to sphinx?
>> That comes down to the "confusion" mentioned at the top, I assume.  It
>> would be good to understand that?
>
> There is no method I have found to discover the -j option's contents
> (intentionally so, it seems) from within make. :(

-- 
Jani Nikula, Intel Open Source Graphics Center