core: add cgroup CPU controller support on the unified hierarchy

Unfortunately, due to the disagreements in the kernel development community,
CPU controller cgroup v2 support has not been merged and enabling it requires
applying two small out-of-tree kernel patches.  The situation is explained in
the following documentation.

 https://git.kernel.org/cgit/linux/kernel/git/tj/cgroup.git/tree/Documentation/cgroup-v2-cpu.txt?h=cgroup-v2-cpu

While it isn't clear what will happen with CPU controller cgroup v2 support,
there are critical features which are possible only on cgroup v2 such as
buffered write control making cgroup v2 essential for a lot of workloads.  This
commit implements systemd CPU controller support on the unified hierarchy so
that users who choose to deploy CPU controller cgroup v2 support can easily
take advantage of it.

On the unified hierarchy, "cpu.weight" knob replaces "cpu.shares" and "cpu.max"
replaces "cpu.cfs_period_us" and "cpu.cfs_quota_us".  [Startup]CPUWeight config
options are added with the usual compat translation.  CPU quota settings remain
unchanged and apply to both legacy and unified hierarchies.

v2: - Error in man page corrected.
    - CPU config application in cgroup_context_apply() refactored.
    - CPU accounting now works on unified hierarchy.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index 47eb8d50a6..a6c69938dd 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -184,8 +184,8 @@ struct Unit {
         UnitFileState unit_file_state;
         int unit_file_preset;
 
-        /* Where the cpuacct.usage cgroup counter was at the time the unit was started */
-        nsec_t cpuacct_usage_base;
+        /* Where the cpu.stat or cpuacct.usage was at the time the unit was started */
+        nsec_t cpu_usage_base;
 
         /* Counterparts in the cgroup filesystem */
         char *cgroup_path;