vchiq: Performance regression since 5.18-rc1

[Stefan Wahren]

Hi,

while testing the staging/vc04_services/interface/vchiq_arm driver with 
my Raspberry Pi 3 B+ (multi_v7_defconfig) i noticed a huge performance 
regression since [ff042f4a9b050895a42cae893cc01fa2ca81b95c] mm: 
lru_cache_disable: replace work queue synchronization with synchronize_rcu

Usually i run "vchiq_test -f 1" to see the driver is still working [1].

Before commit:

real    0m1,500s
user    0m0,068s
sys    0m0,846s

After commit:

real    7m11,449s
user    0m2,049s
sys    0m0,023s

Best regards

[1] - https://github.com/raspberrypi/userland