do_wait: make PIDTYPE_PID case O(1) instead of O(n)

Add a special-case when waiting on a pid (via waitpid, waitid, wait4, etc)
to avoid doing an O(n) scan of children and tracees, 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.

Time to fork and then call waitpid on the child, from a task that already
has N children [1]:

N    | Before  | After
-----|---------|------
1    | 74 us   | 74 us
20   | 72 us   | 75 us
100  | 83 us   | 77 us
500  | 99 us   | 74 us
1000 | 179 us  | 75 us
5000 | 804 us  | 79 us
8000 | 1268 us | 78 us

[1]: https://lkml.org/lkml/2021/3/12/1567

This can make a substantial performance improvement for applications with
a thread that has many children or tracees and frequently needs to wait on
them.  Tools that use ptrace to intercept syscalls for a large number of
processes are likely to fall into this category.  In particular this patch
was developed while building a ptrace-based second generation of the
Shadow emulator [2], for which it allows us to avoid quadratic scaling
(without having to use a workaround that introduces a ~40% performance
penalty) [3].  Other examples of tools that fall into this category which
this patch may help include User Mode Linux [4] and DetTrace [5].

[2]: https://shadow.github.io/
[3]: https://github.com/shadow/shadow/issues/1134#issuecomment-798992292
[4]: https://en.wikipedia.org/wiki/User-mode_Linux
[5]: https://github.com/dettrace/dettrace

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: James Newsome <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Cc: "Eric W . Biederman" <[email protected]>
Cc: Christian Brauner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions