On Thu, Mar 14, 2019 at 10:47:17AM -0700, Joel Fernandes wrote: > About the 100ms latency, I wonder whether it is that high because of > the way Android's lmkd is observing that a process has died. There is > a gap between when a process memory is freed and when it disappears > from the process-table. Once a process is SIGKILLed, it becomes a > zombie. Its memory is freed instantly during the SIGKILL delivery (I > traced this so that's how I know), but until it is reaped by its > parent thread, it will still exist in /proc/<pid> . So if testing the > existence of /proc/<pid> is how Android is observing that the process > died, then there can be a large latency where it takes a very long > time for the parent to actually reap the child way after its memory > was long freed. A quicker way to know if a process's memory is freed > before it is reaped could be to read back /proc/<pid>/maps in > userspace of the victim <pid>, and that file will be empty for zombie > processes. So then one does not need wait for the parent to reap it. I > wonder how much of that 100ms you mentioned is actually the "Waiting > while Parent is reaping the child", than "memory freeing time". So > yeah for this second problem, the procfds work will help. > > By the way another approach that can provide a quick and asynchronous > notification of when the process memory is freed, is to monitor > sched_process_exit trace event using eBPF. You can tell eBPF the PID > that you want to monitor before the SIGKILL. As soon as the process > dies and its memory is freed, the eBPF program can send a notification > to user space (using the perf_events polling infra). The > sched_process_exit fires just after the mmput() happens so it is quite > close to when the memory is reclaimed. This also doesn't need any > kernel changes. I could come up with a prototype for this and > benchmark it on Android, if you want. Just let me know. Perhaps I'm missing something, but if you want to know when a process has died after sending a SIGKILL to it, then why not just make the SIGKILL optionally block until the process has died completely? It'd be rather trivial to just store a pointer to an onstack completion inside the victim process' task_struct, and then complete it in free_task(). Thanks, Sultan _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
