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SubjectRe: [PATCH v5] do_wait: make PIDTYPE_PID case O(1) instead of O(n)
Jim Newsome <jnewsome@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

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