pid: reorder fields in pid_namespace to reduce false sharing

alloc_pid() loads pid_cachep, level and pid_max prior to taking the
lock.

It dirties idr and pid_allocated with the lock.

Some of these fields share the cacheline as is, split them up.

No change in the size of the struct.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20260120204820.1497002-1-mjguzik@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mjguzik

include/linux/pid_namespace.h

@@ -27,6 +27,13 @@ struct pid_namespace {
 	struct idr idr;
 	struct rcu_head rcu;
 	unsigned int pid_allocated;
+#ifdef CONFIG_SYSCTL
+#if defined(CONFIG_MEMFD_CREATE)
+	int memfd_noexec_scope;
+#endif
+	struct ctl_table_set	set;
+	struct ctl_table_header *sysctls;
+#endif
 	struct task_struct *child_reaper;
 	struct kmem_cache *pid_cachep;
 	unsigned int level;
@@ -40,13 +47,6 @@ struct pid_namespace {
 	int reboot;	/* group exit code if this pidns was rebooted */
 	struct ns_common ns;
 	struct work_struct	work;
-#ifdef CONFIG_SYSCTL
-	struct ctl_table_set	set;
-	struct ctl_table_header *sysctls;
-#if defined(CONFIG_MEMFD_CREATE)
-	int memfd_noexec_scope;
-#endif
-#endif
 } __randomize_layout;
 
 extern struct pid_namespace init_pid_ns;