Jim Newsome <jnews...@torproject.org> writes: > do_wait is an internal function used to implement waitpid, waitid, > wait4, etc. To handle the general case, it does an O(n) linear scan of > the thread group's children and tracees. > > This patch adds a special-case when waiting on a pid to skip these scans > and instead do an O(1) lookup. This improves performance when waiting on > a pid from a thread group with many children and/or tracees.
I am going to kibitz just a little bit more. When I looked at this a second time it became apparent that using pid_task twice should actually be faster as it removes a dependent load caused by thread_group_leader, and replaces it by accessing two adjacent pointers in the same cache line. I know the algorithmic improvement is the main advantage, but removing 60ns or so for a dependent load can't hurt. Plus I think using the two pid types really makes it clear that one is always a process and the other is always potentially a thread. /* * Optimization for waiting on PIDTYPE_PID. No need to iterate through child * and tracee lists to find the target task. */ static int do_wait_pid(struct wait_opts *wo) { bool ptrace; struct task_struct *target; int retval; ptrace = false; target = pid_task(wo->wo_pid, PIDTYPE_TGID); if (target && is_effectively_child(wo, ptrace, target)) { retval = wait_consider_task(wo, ptrace, target); if (retval) return retval; } ptrace = true; target = pid_task(wo->wo_pid, PIDTYPE_PID); if (target && target->ptrace && is_effectively_child(wo, ptrace, target)) { retval = wait_consider_task(wo, ptrace, target); if (retval) return retval; } return 0; } Since the probably needs to be respun to include the improved description can we look at my micro performance improvement? Eric