[PATCH v4 0/5] reduce tasklist_lock hold time on exit and do some pid cleanup

Mateusz Guzik
2025-02-05T19:32:53Z

The clone side contends against exit side in a way which avoidably
exacerbates the problem by the latter waiting on locks held by the
former while holding the tasklist_lock.

Whacking this for both add_device_randomness and pids allocation gives
me a 15% speed up for thread creation/destruction in a 24-core vm.

The random patch is worth about 4%.

nothing blew up with lockdep, lightly tested so far

Bench (plop into will-it-scale):
$ cat tests/threadspawn1.c

char *testcase_description = "Thread creation and teardown";

static void *worker(void *arg)
{
return (NULL);
}

void testcase(unsigned long long *iterations, unsigned long nr)
{
pthread_t thread;
int error;

while (1) {
error = pthread_create(&thread, NULL, worker, NULL);
assert(error == 0);
error = pthread_join(thread, NULL);
assert(error == 0);
(*iterations)++;
}
}

v4:
- justify moving get_pid in the commit message with a one-liner
- drop the tty unref patch -- it is completely optional and Oleg has his
own variant
- add the ACK by Oleg

v3:
- keep procfs flush where it was, instead hoist get_pid outside of the
lock
- make detach_pid et al accept an array argument of pids to populate
- sprinkle asserts
- drop irq trips around pidmap_lock
- move tty unref outside of tasklist_lock

Mateusz Guzik (5):
exit: perform add_device_randomness() without tasklist_lock
exit: hoist get_pid() in release_task() outside of tasklist_lock
pid: sprinkle tasklist_lock asserts
pid: perform free_pid() calls outside of tasklist_lock
pid: drop irq disablement around pidmap_lock

include/linux/pid.h | 7 ++-
kernel/exit.c | 36 +++++++----
kernel/pid.c | 82 ++++++++++++++------------
kernel/sys.c | 14 +++--
scripts/selinux/genheaders/genheaders | Bin 90112 -> 0 bytes
5 files changed, 82 insertions(+), 57 deletions(-)
delete mode 100755 scripts/selinux/genheaders/genheaders

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2.43.0