Bug 2140203

Description of problem:
With RHEL 8.6 the tuned throughput-performance profile uses the scheduler plugin for some settings for which it used the sysctl plugin, before (e.g. in RHEL 7.9).

Version-Release number of selected component (if applicable):
tuned-2.18.0-2.el8_6.1.noarch

How reproducible:
always


Steps to Reproduce:
1. make sure that the throughput-performance tuned profile is activated (otherwise: `tuned-adm profile throughput-performance`)
2. increase the fork-rate of the system (until the tuned process uses 30 % CPU or more)
3. `perf trace -s -p $(pgrep tuned) -- sleep 60`

Actual results:
tuned CPU usage increases with the fork rate, easily up to 30 % and more
perf trace output shows high syscall rates for one tuned thread, i.e. for poll(), read(), openat(), lseek(), ioctl(), close() and fstat()

Expected results:
tuned CPU usage is very low (just a few percent) and is independent of the fork rate of the system.

Additional info:
This is caused by how the scheduler plugin polls for process creation events, even when the plugin's usage doesn't contain any process matching declarations, as with the throughput-performance profile. Each such event is then amplified by tuned invoking multiple syscalls on pseudo files under /proc/$pid/.

Looking at a syscall trace in detail shows that a bunch of syscalls to read files under /proc/$pid/ is superfluous or even pointless (even if there were process matching declarations in the config), e.g.:

```
196436 openat(AT_FDCWD, "/proc/3678736/cmdline", O_RDONLY|O_CLOEXEC) = 28</proc/3678736/cmdline>
196436 fstat(28</proc/3678736/cmdline>, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
196436 ioctl(28</proc/3678736/cmdline>, TCGETS, 0x7f1113ffd410) = -1 ENOTTY (Inappropriate ioctl for device)
196436 lseek(28</proc/3678736/cmdline>, 0, SEEK_CUR) = 0
196436 ioctl(28</proc/3678736/cmdline>, TCGETS, 0x7f1113ffd3f0) = -1 ENOTTY (Inappropriate ioctl for device)
196436 lseek(28</proc/3678736/cmdline>, 0, SEEK_CUR) = 0
196436 read(28</proc/3678736/cmdline>, "/opt/xyz/bin/foobar\0foobar\0", 8192) = 23
196436 read(28</proc/3678736/cmdline>, "", 8192) = 0
196436 close(28</proc/3678736/cmdline>) = 0
```


A simple fix for the throughput-performance profile (which is activated, by default, on RHEL systems) is to convert the scheduler plugin settings back to sysctl ones, e.g. like this:

```
--- /usr/lib/tuned/throughput-performance/tuned.conf    2022-06-08 11:48:16.000000000 +0200
+++ new/throughput-performance/tuned.conf  2022-11-04 18:03:05.468461294 +0100
@@ -58,12 +58,11 @@
 # and move them to swap cache
 vm.swappiness=10
 
-[scheduler]
 # ktune sysctl settings for rhel6 servers, maximizing i/o throughput
 #
 # Minimal preemption granularity for CPU-bound tasks:
 # (default: 1 msec#  (1 + ilog(ncpus)), units: nanoseconds)
-sched_min_granularity_ns = 10000000
+kernel.sched_min_granularity_ns = 10000000
 
 # SCHED_OTHER wake-up granularity.
 # (default: 1 msec#  (1 + ilog(ncpus)), units: nanoseconds)
@@ -71,7 +70,7 @@
 # This option delays the preemption effects of decoupled workloads
 # and reduces their over-scheduling. Synchronous workloads will still
 # have immediate wakeup/sleep latencies.
-sched_wakeup_granularity_ns = 15000000
+kernel.sched_wakeup_granularity_ns = 15000000
 
 # Marvell ThunderX
 [sysctl.thunderx]
@@ -81,8 +80,8 @@
 kernel.numa_balancing=0
 
 # AMD
-[scheduler.amd]
-type=scheduler
+[sysctl.amd]
+type=sysctl
 uname_regex=x86_64
 cpuinfo_regex=${amd_cpuinfo_regex}
-sched_migration_cost_ns=5000000
+kernel.sched_migration_cost_ns=5000000
```