Merge patch series "reduce tasklist_lock hold time on exit and do some pid cleanup"

Mateusz Guzik <mjguzik@gmail.com> says:

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%.

The new bottleneck is pidmap_lock itself, with the biggest problem being
the allocation itself taking the lock *twice*.

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)++;
	}
}

* patches from https://lore.kernel.org/r/20250206164415.450051-1-mjguzik@gmail.com:
  pid: drop irq disablement around pidmap_lock
  pid: perform free_pid() calls outside of tasklist_lock
  pid: sprinkle tasklist_lock asserts
  exit: hoist get_pid() in release_task() outside of tasklist_lock
  exit: perform add_device_randomness() without tasklist_lock

Link: https://lore.kernel.org/r/20250206164415.450051-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 98837a1ff0f3..311ecebd7d56 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -101,9 +101,9 @@ extern struct pid *get_task_pid(struct task_struct *task, enum pid_type type);
  * these helpers must be called with the tasklist_lock write-held.
  */
 extern void attach_pid(struct task_struct *task, enum pid_type);
-extern void detach_pid(struct task_struct *task, enum pid_type);
-extern void change_pid(struct task_struct *task, enum pid_type,
-			struct pid *pid);
+void detach_pid(struct pid **pids, struct task_struct *task, enum pid_type);
+void change_pid(struct pid **pids, struct task_struct *task, enum pid_type,
+		struct pid *pid);
 extern void exchange_tids(struct task_struct *task, struct task_struct *old);
 extern void transfer_pid(struct task_struct *old, struct task_struct *new,
 			 enum pid_type);
@@ -129,6 +129,7 @@ extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 			     size_t set_tid_size);
 extern void free_pid(struct pid *pid);
+void free_pids(struct pid **pids);
 extern void disable_pid_allocation(struct pid_namespace *ns);
 
 /*