Date:   Sat, 11 Nov 2017 08:30:37 +0100 (CET)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
cc:     Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org,
        Nicolas Palix <nicolas.palix@...g.fr>,
        linux-kernel@...r.kernel.org, cocci@...teme.lip6.fr
Subject: Re: [Cocci] [PATCH v2] coccinelle: fix parallel build with
 CHECK=scripts/coccicheck



On Fri, 10 Nov 2017, Julia Lawall wrote:

>
>
> On Thu, 9 Nov 2017, Masahiro Yamada wrote:
>
> > The command "make -j8 C=1 CHECK=scripts/coccicheck" produces lots of
> > "coccicheck failed" error messages.
>
> The question is where parallelism should be specified.  Currently, make
> coccicheck picks up the number of cores on the machine and passes that to
> Coccinelle.
>
> OPTIONS="$OPTIONS --jobs $NPROC --chunksize 1"
>
> On my 80 core machine with hyperthreading, this runs 160 jobs in parallel,
> while in practice that degrades the performance as compared to 40 or 80
> cores.
>
> On the other hand, if we use the make command line argument (-j), then we
> will only get parallelism up to the number of semantic patches.  Since
> some finish quickly, there will be a lot of wasted cycles.
>
> The best would be that the user knows what works well for his machine, and
> specifies it on the command line, and then that value gets propagated to
> Coccinelle, eg so that -j8 would cause not 8 semantic patches to run in
> parallel but instead would cause Coccinelle to run one semantic patch on 8
> files in parallel.  But I don't know if that can be done.

Sorry for these fairly nonsensical comments.  make -j is going to consider
every file, then parse and run every semantic patch on that file.  If the
parallelism is pushed down into Coccinelle, each semantic patch will be
parsed only once, and then Coccinelle will choose the files for which it
is relevant.  If indexing is used (idutils, glimpse), then for semantic
patches that focus on specific keywords, Coccinelle will efficiently
ignore files that are not relevant.  I don't think there would be many
cases where make -j would win.  Perhaps it would be possible to detect
its used and abort with an appopriate message?

julia


>
> julia
>
> >
> > I do not know the coccinelle internals, but I guess --jobs does not
> > work well if spatch is invoked from Make running in parallel.
> > Disable --jobs in this case.
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> > ---
> >
> > Changes in v2:
> >   - Grep '-j' instead of '--jobserver-auth'.
> >     '--jobserver-*' is not a stable option flag.
> >     Make 4.2 change '--jobserver-fds' into '--jobserver-auth'
> >   - Add -q option to grep
> >
> >  scripts/coccicheck | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/scripts/coccicheck b/scripts/coccicheck
> > index 040a8b1..8bab11e 100755
> > --- a/scripts/coccicheck
> > +++ b/scripts/coccicheck
> > @@ -70,6 +70,9 @@ if [ "$C" = "1" -o "$C" = "2" ]; then
> >      # Take only the last argument, which is the C file to test
> >      shift $(( $# - 1 ))
> >      OPTIONS="$COCCIINCLUDE $1"
> > +
> > +    # --jobs does not work if Make is running in parallel
> > +    echo $MAKEFLAGS | grep -q -E '(^| )-j' && USE_JOBS="no"
> >  else
> >      ONLINE=0
> >      if [ "$KBUILD_EXTMOD" = "" ] ; then
> > --
> > 2.7.4
> >
> >
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>

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