backports: add threaded Coccinelle spatch support

backports: add threaded Coccinelle spatch support

Turns out Coccinelle has threaded support, it lets you
break down the target code you need to parse for a spatch
file into separate buckets, each bucket handling a subset of
the target files needed to be evaluated. This adds support
for that and shaves down the amount of time it gets to
generate target code by 97.19%.

Using:

mcgrof@drvbp1 ~/backports (git::master)$ time \
	./gentree.py	--verbose \
			--clean \
			--refresh \
			/home/mcgrof/linux-next/ \
			/home/mcgrof/build/next-20140311

Before this patch:

real    19m34.867s
user    18m43.492s
sys     0m21.244s

After this patch by using num_cpus threads:

real    2m42.012s
user    21m53.616s
sys     0m22.748s

After this patch by using 3 * num_cpus threads:

real    1m33.316s
user    24m6.080s
sys     0m28.264s

Using num_cpus threads alone was giving 2m42.012s, its why I
experimented in increasing this. I also experimented with
6 * num_cpus threads as well but the amount of time spent
running didn't reduce, lets keep this at 3 * num_cpus for now
and we can experiment and empirically find the best setting for
any system or just make this configurable.

We had SmPL patch integration first committed via
86d99c9d4 back then we were able to scale to a reasonable
similar time:

mcgrof@drvbp1 ~/backports (git::cocci-added)$ time \
	./gentree.py --verbsose \
		     --clean \
		     --refresh \
		     /home/mcgrof/linux-next
		     /home/mcgrof/build/next-20131113
real    1m1.967s
user    0m49.304s
sys     0m11.880s

For history's purpose here's our run time without Coccinelle on
commit 338c4388cc:

mcgrof@drvbp1 ~/backports (git::before-smpl)$ time \
        ./gentree.py --verbose \
                     --clean \
                     --refresh \
                     /home/mcgrof/linux-next/ \
                     /home/mcgrof/build/next-20131029
real    0m29.639s
user    0m17.508s
sys     0m11.568s

This new changes sets us back on track with the timing for when we first
introduced Coccinelle support we just have a lot 5 Coccinelle patches
now. I hope this shows we should be able to scale with Coccinelle much more
efficiently for collateral evolutions that sprinkle through tons of
files.

Screenshots available using htop before [0] and after the patch [1]
while coccinelle is running, this shows how the system is using
resources much more efficiently when doing spatch evaluation for
patch generation on target code.

[0] http://drvbp1.linux-foundation.org/~mcgrof/images/coccinelle-backports/before-threaded-cocci.png
[1] http://drvbp1.linux-foundation.org/~mcgrof/images/coccinelle-backports/after-threaded-cocci.png

Cc: Peter Senna <peter.senna@gmail.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Gilles Muller <Gilles.Muller@lip6.fr>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>