[PATCH 5.4 085/108] cgroup: Optimize single thread migration

Greg Kroah-Hartman
2022-09-13T15:05:09Z

From: Michal Koutný <mkoutny@suse.com>

[ Upstream commit 9a3284fad42f66bb43629c6716709ff791aaa457 ]

There are reports of users who use thread migrations between cgroups and
they report performance drop after d59cfc09c32a ("sched, cgroup: replace
signal_struct->group_rwsem with a global percpu_rwsem"). The effect is
pronounced on machines with more CPUs.

The migration is affected by forking noise happening in the background,
after the mentioned commit a migrating thread must wait for all
(forking) processes on the system, not only of its threadgroup.

There are several places that need to synchronize with migration:
a) do_exit,
b) de_thread,
c) copy_process,
d) cgroup_update_dfl_csses,
e) parallel migration (cgroup_{proc,thread}s_write).

In the case of self-migrating thread, we relax the synchronization on
cgroup_threadgroup_rwsem to avoid the cost of waiting. d) and e) are
excluded with cgroup_mutex, c) does not matter in case of single thread
migration and the executing thread cannot exec(2) or exit(2) while it is
writing into cgroup.threads. In case of do_exit because of signal
delivery, we either exit before the migration or finish the migration
(of not yet PF_EXITING thread) and die afterwards.

This patch handles only the case of self-migration by writing "0" into
cgroup.threads. For simplicity, we always take cgroup_threadgroup_rwsem
with numeric PIDs.

This change improves migration dependent workload performance similar
to per-signal_struct state.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/cgroup/cgroup-internal.h | 5 +++--
kernel/cgroup/cgroup-v1.c | 5 +++--
kernel/cgroup/cgroup.c | 39 +++++++++++++++++++++++++--------
3 files changed, 36 insertions(+), 13 deletions(-)

diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index 236f290224aae..8dfb2526b3aa2 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -250,9 +250,10 @@ int cgroup_migrate(struct task_struct *leader, bool threadgroup,

int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
bool threadgroup);
-struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup)
+struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
+ bool *locked)
__acquires(&cgroup_threadgroup_rwsem);
-void cgroup_procs_write_finish(struct task_struct *task)
+void cgroup_procs_write_finish(struct task_struct *task, bool locked)
__releases(&cgroup_threadgroup_rwsem);

void cgroup_lock_and_drain_offline(struct cgroup *cgrp);
diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 117d70098cd49..aa7577b189e92 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -498,12 +498,13 @@ static ssize_t __cgroup1_procs_write(struct kernfs_open_file *of,
struct task_struct *task;
const struct cred *cred, *tcred;
ssize_t ret;
+ bool locked;

cgrp = cgroup_kn_lock_live(of->kn, false);
if (!cgrp)
return -ENODEV;

- task = cgroup_procs_write_start(buf, threadgroup);
+ task = cgroup_procs_write_start(buf, threadgroup, &locked);
ret = PTR_ERR_OR_ZERO(task);
if (ret)
goto out_unlock;
@@ -526,7 +527,7 @@ static ssize_t __cgroup1_procs_write(struct kernfs_open_file *of,
ret = cgroup_attach_task(cgrp, task, threadgroup);

out_finish:
- cgroup_procs_write_finish(task);
+ cgroup_procs_write_finish(task, locked);
out_unlock:
cgroup_kn_unlock(of->kn);

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 23f0db2900e4b..bc9ee9a18c1e8 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2856,7 +2856,8 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
return ret;
}

-struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup)
+struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
+ bool *locked)
__acquires(&cgroup_threadgroup_rwsem)
{
struct task_struct *tsk;
@@ -2865,7 +2866,21 @@ struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup)
if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
return ERR_PTR(-EINVAL);

- percpu_down_write(&cgroup_threadgroup_rwsem);
+ /*
+ * If we migrate a single thread, we don't care about threadgroup
+ * stability. If the thread is `current`, it won't exit(2) under our
+ * hands or change PID through exec(2). We exclude
+ * cgroup_update_dfl_csses and other cgroup_{proc,thread}s_write
+ * callers by cgroup_mutex.
+ * Therefore, we can skip the global lock.
+ */
+ lockdep_assert_held(&cgroup_mutex);
+ if (pid || threadgroup) {
+ percpu_down_write(&cgroup_threadgroup_rwsem);
+ *locked = true;
+ } else {
+ *locked = false;
+ }

rcu_read_lock();
if (pid) {
@@ -2896,13 +2911,16 @@ struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup)
goto out_unlock_rcu;

out_unlock_threadgroup:
- percpu_up_write(&cgroup_threadgroup_rwsem);
+ if (*locked) {
+ percpu_up_write(&cgroup_threadgroup_rwsem);
+ *locked = false;
+ }
out_unlock_rcu:
rcu_read_unlock();
return tsk;
}

-void cgroup_procs_write_finish(struct task_struct *task)
+void cgroup_procs_write_finish(struct task_struct *task, bool locked)
__releases(&cgroup_threadgroup_rwsem)
{
struct cgroup_subsys *ss;
@@ -2911,7 +2929,8 @@ void cgroup_procs_write_finish(struct task_struct *task)
/* release reference from cgroup_procs_write_start() */
put_task_struct(task);

- percpu_up_write(&cgroup_threadgroup_rwsem);
+ if (locked)
+ percpu_up_write(&cgroup_threadgroup_rwsem);
for_each_subsys(ss, ssid)
if (ss->post_attach)
ss->post_attach();
@@ -4830,12 +4849,13 @@ static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
struct task_struct *task;
const struct cred *saved_cred;
ssize_t ret;
+ bool locked;

dst_cgrp = cgroup_kn_lock_live(of->kn, false);
if (!dst_cgrp)
return -ENODEV;

- task = cgroup_procs_write_start(buf, true);
+ task = cgroup_procs_write_start(buf, true, &locked);
ret = PTR_ERR_OR_ZERO(task);
if (ret)
goto out_unlock;
@@ -4861,7 +4881,7 @@ static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
ret = cgroup_attach_task(dst_cgrp, task, true);

out_finish:
- cgroup_procs_write_finish(task);
+ cgroup_procs_write_finish(task, locked);
out_unlock:
cgroup_kn_unlock(of->kn);

@@ -4881,6 +4901,7 @@ static ssize_t cgroup_threads_write(struct kernfs_open_file *of,
struct task_struct *task;
const struct cred *saved_cred;
ssize_t ret;
+ bool locked;

buf = strstrip(buf);

@@ -4888,7 +4909,7 @@ static ssize_t cgroup_threads_write(struct kernfs_open_file *of,
if (!dst_cgrp)
return -ENODEV;

- task = cgroup_procs_write_start(buf, false);
+ task = cgroup_procs_write_start(buf, false, &locked);
ret = PTR_ERR_OR_ZERO(task);
if (ret)
goto out_unlock;
@@ -4919,7 +4940,7 @@ static ssize_t cgroup_threads_write(struct kernfs_open_file *of,
ret = cgroup_attach_task(dst_cgrp, task, false);

out_finish:
- cgroup_procs_write_finish(task);
+ cgroup_procs_write_finish(task, locked);
out_unlock:
cgroup_kn_unlock(of->kn);

--
2.35.1