[PATCH] Cygwin: Carry process affinity through to result

[Christian Franke]

Mark Geisert wrote:
> Due to deficient testing, the current code doesn't return a valid result
> to users of sched_getaffinity().  Carry the result procmask through to
> the generation of result cpu mask.
>
> Recognize Windows' limitation that if the process is multi-group (i.e.,
> has threads in multiple cpu groups) there is no visibility to which
> processors in other groups are being used.  One could remedy this by
> looping through all the process' threads, but that could be expensive
> so is left for future contemplation.
>
> Reported-by: Christian Franke <Christian.Franke@t-online.de>
> Addresses: https://cygwin.com/pipermail/cygwin/2025-March/257616.html
> Signed-off-by: Mark Geisert <mark@maxrnd.com>
> Fixes: 641ecb07533e ("Cygwin: Implement sched_[gs]etaffinity()")

Could only test the single cpu group (aka single physical cpu) case 
which is the most common, I guess. Works as expected:

$ uname -r
3.6.0-dev-440-g5ec497dc80bc-dirty.x86_64

$ grep '^model name' /proc/cpuinfo | uniq -c
      28 model name      : Intel(R) Core(TM) i7-14700K

$ stress-ng --pthread 1 -v &
[1] 1323
...
stress-ng: debug: [1324] pthread: [1324] started (instance 0 on CPU 10)

$ taskset -c -p 1324
pid 1324's current affinity list: 0-27

$ taskset -p fff0000 1324 # All E-cores
pid 1324's current affinity mask: fffffff
pid 1324's new affinity mask: fff0000

$ taskset -p fff5555 1324 # All cores but no HT
pid 1324's current affinity mask: fff0000
pid 1324's new affinity mask: fff5555

$ taskset -c -p 8,9 1324 # P-core 4 with HT
pid 1324's current affinity list: 0,2,4,6,8,10,12,14,16-27
pid 1324's new affinity list: 8,9

$ taskset -p 1324
pid 1324's current affinity mask: 300

The settings have the desired effect on reported core usage.

-- 
Thanks,
Christian