Add CAP_PERFMON to AmbientCapabilities in capabilities.conf, to enable
perf_event based stall detector in Seastar.
However, on Debian/Ubuntu CAP_PERFMON with non-root user does not work
because it sets kernel.perf_event_paranoid=4 which disallow all non-root
user access.
(On Debian it kernel.perf_event_paranoid=3)
So we need to configure kernel.perf_event_paranoid=2 on these distros.
see: https://askubuntu.com/questions/1400874/what-does-perf-paranoia-level-four-do
Also, CAP_PERFMON is only available on linux-5.8+, older kernel does not
have this capability.
To enable older kernel environment such as CentOS7, we need to configure
kernel.perf_event_paranoid=1 to allow non-root user access even without
the capability.
Fixes#15743Closesscylladb/scylladb#16070
On /usr/lib/sysctl.d/99-scylla-sched.conf, we have some sysctl settings to
tune the scheduler for lower latency.
This is mostly to prevent softirq threads processing tcp and reactor threads
from injecting latency into each other.
However, these parameters are moved to debugfs from linux-5.13+, so we lost
scheduler tuneing on recent kernels.
To support tuning recent kernel, let's add a new service which support
to configure both sysctl and debugfs.
The service named scylla-tune-sched.service
The service will unconditionally enables when installed, on older kernel
it will tune via sysctl, on recent kernel it will tune via debugfs.
Fixes#16077Closesscylladb/scylladb#16122
