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.
>
> Signed-off-by: James Newsome <jnews...@torproject.org>
> ---
> kernel/exit.c | 53 +++++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 43 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 04029e35e69a..c2438d4ba262 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -1439,9 +1439,34 @@ void __wake_up_parent(struct task_struct *p, struct
> task_struct *parent)
> TASK_INTERRUPTIBLE, p);
> }
>
> +// Optimization for waiting on PIDTYPE_PID. No need to iterate through child
> +// and tracee lists to find the target task.
Minor nit: C++ style comments look very out of place in this file
which uses old school C /* */ comment delimiters for
all of it's block comments.
> +static int do_wait_pid(struct wait_opts *wo)
> +{
> + struct task_struct *target = pid_task(wo->wo_pid, PIDTYPE_PID);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This is subtle change in behavior.
Today on the task->children list we only place thread group leaders.
Which means that your do_wait_pid wait for thread of someone else's
process and that is a change in behavior.
So the code either needs a thread_group_leader filter on target before
the ptrace=0 case or we need to use "pid_task(wo->wo_pid, PIDTYPE_TGID)"
and "pid_task(wo->wo_pid, PIDTYPE_PID)" for the "ptrace=1" case.
I would like to make thread_group_leaders go away so I would favor two
pid_task calls. But either will work right now.
Eric
> + int retval;
> +
> + if (!target)
> + return 0;
> + if (current == target->real_parent ||
> + (!(wo->wo_flags & __WNOTHREAD) &&
> + same_thread_group(current, target->real_parent))) {
> + retval = wait_consider_task(wo, /* ptrace= */ 0, target);
> + if (retval)
> + return retval;
> + }
> + if (target->ptrace && (current == target->parent ||
> + (!(wo->wo_flags & __WNOTHREAD) &&
> + same_thread_group(current, target->parent)))) {
> + retval = wait_consider_task(wo, /* ptrace= */ 1, target);
> + if (retval)
> + return retval;
> + }
> + return 0;
> +}
> +
> static long do_wait(struct wait_opts *wo)
> {
> - struct task_struct *tsk;
> int retval;
>
> trace_sched_process_wait(wo->wo_pid);
> @@ -1463,19 +1488,27 @@ static long do_wait(struct wait_opts *wo)
>
> set_current_state(TASK_INTERRUPTIBLE);
> read_lock(&tasklist_lock);
> - tsk = current;
> - do {
> - retval = do_wait_thread(wo, tsk);
> - if (retval)
> - goto end;
>
> - retval = ptrace_do_wait(wo, tsk);
> + if (wo->wo_type == PIDTYPE_PID) {
> + retval = do_wait_pid(wo);
> if (retval)
> goto end;
> + } else {
> + struct task_struct *tsk = current;
>
> - if (wo->wo_flags & __WNOTHREAD)
> - break;
> - } while_each_thread(current, tsk);
> + do {
> + retval = do_wait_thread(wo, tsk);
> + if (retval)
> + goto end;
> +
> + retval = ptrace_do_wait(wo, tsk);
> + if (retval)
> + goto end;
> +
> + if (wo->wo_flags & __WNOTHREAD)
> + break;
> + } while_each_thread(current, tsk);
> + }
> read_unlock(&tasklist_lock);
>
> notask:
