Lennart Poettering
50be4f4a46
This reworks how systemd tracks processes on cgroupv1 systems where cgroup notification is not reliable. Previously, whenever we had reason to believe that new processes showed up or got removed we'd scan the cgroup of the scope or service unit for new processes, and would tidy up the list of PIDs previously watched. This scanning is relatively slow, and does not scale well. With this change behaviour is changed: instead of scanning for new/removed processes right away we do this work in a per-unit deferred event loop job. This event source is scheduled at a very low priority, so that it is executed when we have time but does not starve other event sources. This has two benefits: this expensive work is coalesced, if events happen in quick succession, and we won't delay SIGCHLD handling for too long. This patch basically replaces all direct invocation of unit_watch_all_pids() in scope.c and service.c with invocations of the new unit_enqueue_rewatch_pids() call which just enqueues a request of watching/tidying up the PID sets (with one exception: in scope_enter_signal() and service_enter_signal() we'll still do unit_watch_all_pids() synchronously first, since we really want to know all processes we are about to kill so that we can track them properly. Moreover, all direct invocations of unit_tidy_watch_pids() and unit_synthesize_cgroup_empty_event() are removed too, when the unit_enqueue_rewatch_pids() call is invoked, as the queued job will run those operations too. All of this is done on cgroupsv1 systems only, and is disabled on cgroupsv2 systems as cgroup-empty notifications are reliable there, and we do not need SIGCHLD events to track processes there. Fixes: #9138
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