[PATCH v3 4/5] pid: mark pids associated with group leader tasks

[Brian Foster <bfoster@redhat.com>]

Searching the pid_namespace for group leader tasks is a fairly
inefficient operation. Listing the root directory of a procfs mount
performs a linear scan of allocated pids, checking each entry for an
associated PIDTYPE_TGID task to determine whether to populate a
directory entry. This can cause a significant increase in readdir()
syscall latency when run in namespaces that might have one or more
processes with significant thread counts.

To facilitate improved TGID pid searches, mark the ids of pid
entries that are likely to have an associated PIDTYPE_TGID task. To
keep the code simple and avoid having to maintain synchronization
between mark state and post-fork pid-task association changes, the
mark is applied to all pids allocated for tasks cloned without
CLONE_THREAD.

This means that it is possible for a pid to remain marked in the
xarray after being disassociated from the group leader task. For
example, a process that does a setsid() followed by fork() and
exit() (to daemonize) will remain associated with the original pid
for the session, but link with the child pid as the group leader.
OTOH, the only place other than fork() where a tgid association
occurs is in the exec() path, which kills all other tasks in the
group and associates the current task with the preexisting leader
pid. Therefore, the semantics of the mark are that false positives
(marked pids without PIDTYPE_TGID tasks) are possible, but false
negatives (unmarked pids without PIDTYPE_TGID tasks) should never
occur.

This is an effective optimization because false negatives are fairly
uncommon and don't add overhead (i.e. we already have to check
pid_task() for marked entries), but still filters out thread pids
that are guaranteed not to have TGID task association.

Mark entries in the pid allocation path when the caller specifies
that the pid associates with a new thread group. Since false
negatives are not allowed, warn in the event that a PIDTYPE_TGID
task is ever attached to an unmarked pid. Finally, create a helper
to implement the task search based on the mark semantics defined
above (based on search logic currently implemented by next_tgid() in
procfs).

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
 include/linux/pid.h |  3 ++-
 kernel/fork.c       |  2 +-
 kernel/pid.c        | 44 +++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 343abf22092e..64caf21be256 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -132,9 +132,10 @@ extern struct pid *find_vpid(int nr);
  */
 extern struct pid *find_get_pid(int nr);
 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
+struct task_struct *find_get_tgid_task(int *id, struct pid_namespace *);
 
 extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
-			     size_t set_tid_size);
+			     size_t set_tid_size, bool group_leader);
 extern void free_pid(struct pid *pid);
 extern void disable_pid_allocation(struct pid_namespace *ns);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 08969f5aa38d..1cf2644c642e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2267,7 +2267,7 @@ static __latent_entropy struct task_struct *copy_process(
 
 	if (pid != &init_struct_pid) {
 		pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid,
-				args->set_tid_size);
+				args->set_tid_size, !(clone_flags & CLONE_THREAD));
 		if (IS_ERR(pid)) {
 			retval = PTR_ERR(pid);
 			goto bad_fork_cleanup_thread;
diff --git a/kernel/pid.c b/kernel/pid.c
index 53db06f9882d..d65f74c6186c 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -66,6 +66,9 @@ int pid_max = PID_MAX_DEFAULT;
 int pid_max_min = RESERVED_PIDS + 1;
 int pid_max_max = PID_MAX_LIMIT;
 
+/* MARK_0 used by XA_FREE_MARK */
+#define TGID_MARK	XA_MARK_1
+
 struct pid_namespace init_pid_ns = {
 	.ns.count = REFCOUNT_INIT(2),
 	.xa = XARRAY_INIT(init_pid_ns.xa, PID_XA_FLAGS),
@@ -137,7 +140,7 @@ void free_pid(struct pid *pid)
 }
 
 struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
-		      size_t set_tid_size)
+		      size_t set_tid_size, bool group_leader)
 {
 	struct pid *pid;
 	enum pid_type type;
@@ -257,6 +260,8 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 
 		/* Make the PID visible to find_pid_ns. */
 		__xa_store(&tmp->xa, upid->nr, pid, 0);
+		if (group_leader)
+			__xa_set_mark(&tmp->xa, upid->nr, TGID_MARK);
 		tmp->pid_allocated++;
 		xa_unlock_irq(&tmp->xa);
 	}
@@ -314,6 +319,11 @@ static struct pid **task_pid_ptr(struct task_struct *task, enum pid_type type)
 void attach_pid(struct task_struct *task, enum pid_type type)
 {
 	struct pid *pid = *task_pid_ptr(task, type);
+	struct pid_namespace *pid_ns = ns_of_pid(pid);
+	pid_t pid_nr = pid_nr_ns(pid, pid_ns);
+
+	WARN_ON(type == PIDTYPE_TGID &&
+		!xa_get_mark(&pid_ns->xa, pid_nr, TGID_MARK));
 	hlist_add_head_rcu(&task->pid_links[type], &pid->tasks[type]);
 }
 
@@ -506,6 +516,38 @@ struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 }
 EXPORT_SYMBOL_GPL(find_ge_pid);
 
+/*
+ * Used by proc to find the first thread group leader task with an id greater
+ * than or equal to *id.
+ *
+ * Use the xarray mark as a hint to find the next best pid. The mark does not
+ * guarantee a linked group leader task exists, so retry until a suitable entry
+ * is found.
+ */
+struct task_struct *find_get_tgid_task(int *id, struct pid_namespace *ns)
+{
+	struct pid *pid;
+	struct task_struct *t;
+	unsigned long nr = *id;
+
+	rcu_read_lock();
+	do {
+		pid = xa_find(&ns->xa, &nr, ULONG_MAX, TGID_MARK);
+		if (!pid) {
+			rcu_read_unlock();
+			return NULL;
+		}
+		t = pid_task(pid, PIDTYPE_TGID);
+		nr++;
+	} while (!t);
+
+	*id = pid_nr_ns(pid, ns);
+	get_task_struct(t);
+	rcu_read_unlock();
+
+	return t;
+}
+
 struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
 {
 	struct fd f;
-- 
2.37.3