manager: Only invoke a single sigchld per unit within a cleanup cycle

By default, each iteration of manager_dispatch_sigchld() results in a unit level
sigchld event being invoked. For scope units, this results in a scope_sigchld_event()
which can seemingly stall for workloads that have a large number of PIDs within the
scope. The stall exhibits itself as a SIG_0 being initiated for each u->pids entry
as a result of pid_is_unwaited().

v2:
This patch resolves this condition by only paying to cost of a sigchld in the underlying
scope unit once per sigchld iteration. A new "sigchldgen" member resides within the
Unit struct. The Manager is incremented via the sd event loop, accessed via
sd_event_get_iteration, and the Unit member is set to the same value as the manager each
time that a sigchld event is invoked. If the Manager iteration value and Unit member
match, the sigchld event is not invoked for that iteration.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index 08a927962d..c41011ed9d 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -162,6 +162,9 @@ struct Unit {
          * process SIGCHLD for */
         Set *pids;
 
+        /* Used in sigchld event invocation to avoid repeat events being invoked */
+        uint64_t sigchldgen;
+
         /* Used during GC sweeps */
         unsigned gc_marker;