[PATCH] Makefile: fix bugs in coccicheck and speed it up

I've often wondered why "make coccicheck" takes so long. This change speeds it up by ~2x and makes it use much less memory. Or a reduction of a max of around ~2GB per-process (under the old SPATCH_BATCH_SIZE=0) to around ~200MB.

Running the full "make coccicheck" now takes ~50 seconds with -j8 on my machine, v.s. ~2x of that before. I've got 64GB of memory on that machine, or it would be much slower.

Why has it been so slow? Because I think we've always been running it in entirely the wrong mode for what we wanted, and much of the previous fixing of this target has involved re-arranging the deck chairs on that particular Titanic.

What we really want to do with coccicheck is to do search/replacements in all our *.c and *.h files. This is now what we do, and we'll process a default of 64 files at a time.

What we were doing before was processing all our *.c files, and for each of those *.c files we'd recursively look around for includes and see if we needed to search/replace in those too.

That we did that dates back to [1] when we were only processing *.c files, and it was always very redundant. We'd e.g. visit the likes of strbuf.h lots of times since it's widely used as an include.

Then in the most recent attempt to optimize coccicheck in [2] this anti-pattern finally turned into a bug.

Namely: before this change, if your coccicheck rule applied to
e.g. making a change in strbuf.h itself we'd get *lots* of duplicate
hunks applying the exact same change, as concurrent spatch processes
invoked by xargs raced one another. In one instance I ended up with 27
copies of the same hunk in a strbuf.patch.

Setting SPATCH_BATCH_SIZE=0 and processing all the files in one giant batch mitigated this. I suspect the author of [2] either mostly ran in that mode, or didn't test on changes that impacted widely used header files.

So since we're going to want to process all our *.c and *.h let's just do that, and drop --all-includes for --no-includes. It's not spatch's job to find our sources, we're doing that. If someone is manually tweaking COCCI_SOURCES they can just tweak SPATCH_FLAGS too.

I'm entirely removing SPATCH_BATCH_SIZE. If you want to tweak it you can tweak SPATCH_XARGS_FLAGS to e.g. "-n 256", or "-P 4 -n 128". But in my testing it isn't worth it to tweak SPATCH_XARGS_FLAGS for a full "make coccicheck".

I'm also the whole "cat $@.log" introduced in [3]. Since we don't call this in a loop anymore (and xargs will early-exit) we can just rely on standard V=1 for debugging issues.

1. a9a884aea5 (coccicheck: use --all-includes by default, 2016-09-30)
2. 960154b9c1 (coccicheck: optionally batch spatch invocations,
   2019-05-06)
3. f5c2bc2b96 (Makefile: detect errors in running spatch, 2017-03-10)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
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 Makefile | 41 ++++++++++++++++++++++-------------------
 1 file changed, 22 insertions(+), 19 deletions(-)