[PATCH v9 5/8] cgroup/cpuset: convert cpuset_mutex to percpu_rwsem

Juri Lelli
2019-07-19T13:59:57Z

Holding cpuset_mutex means that cpusets are stable (only the holder can
make changes) and this is required for fixing a synchronization issue
between cpusets and scheduler core. However, grabbing cpuset_mutex from
setscheduler() hotpath (as implemented in a later patch) is a no-go, as
it would create a bottleneck for tasks concurrently calling
setscheduler().

Convert cpuset_mutex to be a percpu_rwsem (cpuset_rwsem), so that
setscheduler() will then be able to read lock it and avoid concurrency
issues.

Signed-off-by: Juri Lelli <juri.lelli@redhat.com>

---
v8 -> v9:
- make cpuset_{can,cancel}_attach grab cpuset_rwsem for write (Peter)
---
kernel/cgroup/cpuset.c | 68 ++++++++++++++++++++++--------------------
1 file changed, 35 insertions(+), 33 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 48d29a6112cb..85491d09f3d3 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -333,7 +333,7 @@ static struct cpuset top_cpuset = {
* guidelines for accessing subsystem state in kernel/cgroup.c
*/

-static DEFINE_MUTEX(cpuset_mutex);
+DEFINE_STATIC_PERCPU_RWSEM(cpuset_rwsem);
static DEFINE_SPINLOCK(callback_lock);

static struct workqueue_struct *cpuset_migrate_mm_wq;
@@ -966,7 +966,7 @@ static void rebuild_root_domains(void)
struct cpuset *cs = NULL;
struct cgroup_subsys_state *pos_css;

- lockdep_assert_held(&cpuset_mutex);
+ percpu_rwsem_assert_held(&cpuset_rwsem);
lockdep_assert_cpus_held();
lockdep_assert_held(&sched_domains_mutex);

@@ -1026,7 +1026,7 @@ static void rebuild_sched_domains_locked(void)
cpumask_var_t *doms;
int ndoms;

- lockdep_assert_held(&cpuset_mutex);
+ percpu_rwsem_assert_held(&cpuset_rwsem);
get_online_cpus();

/*
@@ -1058,9 +1058,9 @@ static void rebuild_sched_domains_locked(void)

void rebuild_sched_domains(void)
{
- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);
rebuild_sched_domains_locked();
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
}

/**
@@ -1166,7 +1166,7 @@ static int update_parent_subparts_cpumask(struct cpuset *cpuset, int cmd,
int deleting; /* Moving cpus from subparts_cpus to effective_cpus */
bool part_error = false; /* Partition error? */

- lockdep_assert_held(&cpuset_mutex);
+ percpu_rwsem_assert_held(&cpuset_rwsem);

/*
* The parent must be a partition root.
@@ -2154,7 +2154,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
cpuset_attach_old_cs = task_cs(cgroup_taskset_first(tset, &css));
cs = css_cs(css);

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);

/* allow moving tasks into an empty cpuset if on default hierarchy */
ret = -ENOSPC;
@@ -2178,7 +2178,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
cs->attach_in_progress++;
ret = 0;
out_unlock:
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
return ret;
}

@@ -2188,9 +2188,9 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)

cgroup_taskset_first(tset, &css);

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);
css_cs(css)->attach_in_progress--;
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
}

/*
@@ -2213,7 +2213,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
cgroup_taskset_first(tset, &css);
cs = css_cs(css);

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);

/* prepare for attach */
if (cs == &top_cpuset)
@@ -2267,7 +2267,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
if (!cs->attach_in_progress)
wake_up(&cpuset_attach_wq);

- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
}

/* The various types of files and directories in a cpuset file system */
@@ -2298,7 +2298,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
cpuset_filetype_t type = cft->private;
int retval = 0;

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);
if (!is_cpuset_online(cs)) {
retval = -ENODEV;
goto out_unlock;
@@ -2334,7 +2334,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
break;
}
out_unlock:
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
return retval;
}

@@ -2345,7 +2345,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
cpuset_filetype_t type = cft->private;
int retval = -ENODEV;

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);
if (!is_cpuset_online(cs))
goto out_unlock;

@@ -2358,7 +2358,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
break;
}
out_unlock:
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
return retval;
}

@@ -2397,7 +2397,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
kernfs_break_active_protection(of->kn);
flush_work(&cpuset_hotplug_work);

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);
if (!is_cpuset_online(cs))
goto out_unlock;

@@ -2421,7 +2421,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,

free_cpuset(trialcs);
out_unlock:
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
kernfs_unbreak_active_protection(of->kn);
css_put(&cs->css);
flush_workqueue(cpuset_migrate_mm_wq);
@@ -2552,13 +2552,13 @@ static ssize_t sched_partition_write(struct kernfs_open_file *of, char *buf,
return -EINVAL;

css_get(&cs->css);
- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);
if (!is_cpuset_online(cs))
goto out_unlock;

retval = update_prstate(cs, val);
out_unlock:
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
css_put(&cs->css);
return retval ?: nbytes;
}
@@ -2764,7 +2764,7 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
if (!parent)
return 0;

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);

set_bit(CS_ONLINE, &cs->flags);
if (is_spread_page(parent))
@@ -2815,7 +2815,7 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
cpumask_copy(cs->effective_cpus, parent->cpus_allowed);
spin_unlock_irq(&callback_lock);
out_unlock:
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
return 0;
}

@@ -2834,7 +2834,7 @@ static void cpuset_css_offline(struct cgroup_subsys_state *css)
{
struct cpuset *cs = css_cs(css);

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);

if (is_partition_root(cs))
update_prstate(cs, 0);
@@ -2853,7 +2853,7 @@ static void cpuset_css_offline(struct cgroup_subsys_state *css)
cpuset_dec();
clear_bit(CS_ONLINE, &cs->flags);

- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
}

static void cpuset_css_free(struct cgroup_subsys_state *css)
@@ -2865,7 +2865,7 @@ static void cpuset_css_free(struct cgroup_subsys_state *css)

static void cpuset_bind(struct cgroup_subsys_state *root_css)
{
- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);
spin_lock_irq(&callback_lock);

if (is_in_v2_mode()) {
@@ -2878,7 +2878,7 @@ static void cpuset_bind(struct cgroup_subsys_state *root_css)
}

spin_unlock_irq(&callback_lock);
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
}

/*
@@ -2922,6 +2922,8 @@ int __init cpuset_init(void)
{
int err = 0;

+ BUG_ON(percpu_init_rwsem(&cpuset_rwsem));
+
BUG_ON(!alloc_cpumask_var(&top_cpuset.cpus_allowed, GFP_KERNEL));
BUG_ON(!alloc_cpumask_var(&top_cpuset.effective_cpus, GFP_KERNEL));
BUG_ON(!zalloc_cpumask_var(&top_cpuset.subparts_cpus, GFP_KERNEL));
@@ -2997,7 +2999,7 @@ hotplug_update_tasks_legacy(struct cpuset *cs,
is_empty = cpumask_empty(cs->cpus_allowed) ||
nodes_empty(cs->mems_allowed);

- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);

/*
* Move tasks to the nearest ancestor with execution resources,
@@ -3007,7 +3009,7 @@ hotplug_update_tasks_legacy(struct cpuset *cs,
if (is_empty)
remove_tasks_in_empty_cpuset(cs);

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);
}

static void
@@ -3057,14 +3059,14 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
retry:
wait_event(cpuset_attach_wq, cs->attach_in_progress == 0);

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);

/*
* We have raced with task attaching. We wait until attaching
* is finished, so we won't attach a task to an empty cpuset.
*/
if (cs->attach_in_progress) {
- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
goto retry;
}

@@ -3132,7 +3134,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
hotplug_update_tasks_legacy(cs, &new_cpus, &new_mems,
cpus_updated, mems_updated);

- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);
}

/**
@@ -3162,7 +3164,7 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
if (on_dfl && !alloc_cpumasks(NULL, &tmp))
ptmp = &tmp;

- mutex_lock(&cpuset_mutex);
+ percpu_down_write(&cpuset_rwsem);

/* fetch the available cpus/mems and find out which changed how */
cpumask_copy(&new_cpus, cpu_active_mask);
@@ -3212,7 +3214,7 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
update_tasks_nodemask(&top_cpuset);
}

- mutex_unlock(&cpuset_mutex);
+ percpu_up_write(&cpuset_rwsem);

/* if cpus or mems changed, we need to propagate to descendants */
if (cpus_updated || mems_updated) {
--
2.17.2