Remove circular dependency deadlock in a scenario where hotplug of CPU is
being done while there is updation in cgroup and cpuset triggered from
userspace.
Example scenario:
kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0
kworker/0:0 - percpu_down_write(&cpu_hotplug_lock) [held]
flush(work) [no high prio workqueue available on CPU]
wait_for_completion()
kthreadd - percpu_down_read(cgroup_threadgroup_rwsem) [waiting]
init:729 - percpu_down_write(cgroup_threadgroup_rwsem) [held]
lock(cpuset_mutex) [waiting]
init:1 - lock(cpuset_mutex) [held]
percpu_down_read(&cpu_hotplug_lock) [waiting]
Eliminate this dependecy by reordering locking of cpuset_mutex
and cpu_hotplug_lock in following order
1. Acquire cpu_hotplug_lock (read)
2. Acquire cpuset_mutex
Signed-off-by: Prateek Sood <[email protected]>
---
kernel/cgroup/cpuset.c | 70 +++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 61 insertions(+), 9 deletions(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 2f4039b..687be57 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -843,10 +843,41 @@ static void rebuild_sched_domains_locked(void)
out:
put_online_cpus();
}
+
+/*
+ * Rebuild scheduler domains.
+ * Call with following lock held in the order
+ * 1. cpu_hotplug_lock (read)
+ * 2. cpuset_mutex
+ */
+static void rebuild_sched_domains_unlocked(void)
+{
+ struct sched_domain_attr *attr;
+ cpumask_var_t *doms;
+ int ndoms;
+
+ /*
+ * We have raced with CPU hotplug. Don't do anything to avoid
+ * passing doms with offlined cpu to partition_sched_domains().
+ * Anyways, hotplug work item will rebuild sched domains.
+ */
+ if (!cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
+ return;
+
+ /* Generate domain masks and attrs */
+ ndoms = generate_sched_domains(&doms, &attr);
+
+ /* Have scheduler rebuild the domains */
+ partition_sched_domains(ndoms, doms, attr);
+}
#else /* !CONFIG_SMP */
static void rebuild_sched_domains_locked(void)
{
}
+
+static void rebuild_sched_domains_unlocked(void)
+{
+}
#endif /* CONFIG_SMP */
void rebuild_sched_domains(void)
@@ -885,7 +916,9 @@ static void update_tasks_cpumask(struct cpuset *cs)
*
* On legacy hierachy, effective_cpus will be the same with cpu_allowed.
*
- * Called with cpuset_mutex held
+ * Called with following lock held in order
+ * 1. cpu_hotplug_lock (read)
+ * 2. cpuset_mutex
*/
static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
{
@@ -940,7 +973,7 @@ static void update_cpumasks_hier(struct cpuset *cs, struct cpumask *new_cpus)
rcu_read_unlock();
if (need_rebuild_sched_domains)
- rebuild_sched_domains_locked();
+ rebuild_sched_domains_unlocked();
}
/**
@@ -1262,6 +1295,11 @@ int current_cpuset_is_being_rebound(void)
return ret;
}
+/*
+ * Call with following lock held in order
+ * 1. cpu_hotplug_lock (read)
+ * 2. cpuset_mutex
+ */
static int update_relax_domain_level(struct cpuset *cs, s64 val)
{
#ifdef CONFIG_SMP
@@ -1273,7 +1311,7 @@ static int update_relax_domain_level(struct cpuset *cs, s64 val)
cs->relax_domain_level = val;
if (!cpumask_empty(cs->cpus_allowed) &&
is_sched_load_balance(cs))
- rebuild_sched_domains_locked();
+ rebuild_sched_domains_unlocked();
}
return 0;
@@ -1304,9 +1342,10 @@ static void update_tasks_flags(struct cpuset *cs)
* cs: the cpuset to update
* turning_on: whether the flag is being set or cleared
*
- * Call with cpuset_mutex held.
+ * Call with following lock held in order
+ * 1. cpu_hotplug_lock (read)
+ * 2. cpuset_mutex
*/
-
static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
int turning_on)
{
@@ -1339,7 +1378,7 @@ static int update_flag(cpuset_flagbits_t bit, struct cpuset *cs,
spin_unlock_irq(&callback_lock);
if (!cpumask_empty(trialcs->cpus_allowed) && balance_flag_changed)
- rebuild_sched_domains_locked();
+ rebuild_sched_domains_unlocked();
if (spread_flag_changed)
update_tasks_flags(cs);
@@ -1607,6 +1646,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
cpuset_filetype_t type = cft->private;
int retval = 0;
+ get_online_cpus();
mutex_lock(&cpuset_mutex);
if (!is_cpuset_online(cs)) {
retval = -ENODEV;
@@ -1644,6 +1684,7 @@ static int cpuset_write_u64(struct cgroup_subsys_state *css, struct cftype *cft,
}
out_unlock:
mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
return retval;
}
@@ -1654,6 +1695,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
cpuset_filetype_t type = cft->private;
int retval = -ENODEV;
+ get_online_cpus();
mutex_lock(&cpuset_mutex);
if (!is_cpuset_online(cs))
goto out_unlock;
@@ -1668,6 +1710,7 @@ static int cpuset_write_s64(struct cgroup_subsys_state *css, struct cftype *cft,
}
out_unlock:
mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
return retval;
}
@@ -1706,6 +1749,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
kernfs_break_active_protection(of->kn);
flush_work(&cpuset_hotplug_work);
+ get_online_cpus();
mutex_lock(&cpuset_mutex);
if (!is_cpuset_online(cs))
goto out_unlock;
@@ -1731,6 +1775,7 @@ static ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
free_trial_cpuset(trialcs);
out_unlock:
mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
kernfs_unbreak_active_protection(of->kn);
css_put(&cs->css);
flush_workqueue(cpuset_migrate_mm_wq);
@@ -2031,13 +2076,14 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
/*
* If the cpuset being removed has its flag 'sched_load_balance'
* enabled, then simulate turning sched_load_balance off, which
- * will call rebuild_sched_domains_locked().
+ * will call rebuild_sched_domains_unlocked().
*/
static void cpuset_css_offline(struct cgroup_subsys_state *css)
{
struct cpuset *cs = css_cs(css);
+ get_online_cpus();
mutex_lock(&cpuset_mutex);
if (is_sched_load_balance(cs))
@@ -2047,6 +2093,7 @@ static void cpuset_css_offline(struct cgroup_subsys_state *css)
clear_bit(CS_ONLINE, &cs->flags);
mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
}
static void cpuset_css_free(struct cgroup_subsys_state *css)
@@ -2341,8 +2388,13 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
}
/* rebuild sched domains if cpus_allowed has changed */
- if (cpus_updated)
- rebuild_sched_domains();
+ if (cpus_updated) {
+ get_online_cpus();
+ mutex_lock(&cpuset_mutex);
+ rebuild_sched_domains_unlocked();
+ mutex_unlock(&cpuset_mutex);
+ put_online_cpus();
+ }
}
void cpuset_update_active_cpus(void)
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.,
is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
On Thu, Sep 07, 2017 at 11:34:12AM +0530, Prateek Sood wrote:
> Remove circular dependency deadlock in a scenario where hotplug of CPU is
> being done while there is updation in cgroup and cpuset triggered from
> userspace.
>
> Example scenario:
> kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0
>
> kworker/0:0 - percpu_down_write(&cpu_hotplug_lock) [held]
> flush(work) [no high prio workqueue available on CPU]
> wait_for_completion()
>
> kthreadd - percpu_down_read(cgroup_threadgroup_rwsem) [waiting]
>
> init:729 - percpu_down_write(cgroup_threadgroup_rwsem) [held]
> lock(cpuset_mutex) [waiting]
>
> init:1 - lock(cpuset_mutex) [held]
> percpu_down_read(&cpu_hotplug_lock) [waiting]
That's both unreadable and useless :/ You want to tell what code paths
that were, not which random tasks happened to run them.
> Eliminate this dependecy by reordering locking of cpuset_mutex
> and cpu_hotplug_lock in following order
> 1. Acquire cpu_hotplug_lock (read)
> 2. Acquire cpuset_mutex
>
> Signed-off-by: Prateek Sood <[email protected]>
> ---
> kernel/cgroup/cpuset.c | 70 +++++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 61 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 2f4039b..687be57 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -843,10 +843,41 @@ static void rebuild_sched_domains_locked(void)
> out:
> put_online_cpus();
> }
> +
> +/*
> + * Rebuild scheduler domains.
> + * Call with following lock held in the order
> + * 1. cpu_hotplug_lock (read)
> + * 2. cpuset_mutex
Do not put that in comments, nobody ever reads comments.
> + */
> +static void rebuild_sched_domains_unlocked(void)
The common postfix for a function called with the cpuhotplug lock held
is: _cpuslocked()
> +{
> + struct sched_domain_attr *attr;
> + cpumask_var_t *doms;
> + int ndoms;
lockdep_assert_cpus_held();
lockdep_assert_held(&cpuset_mutex);
> +
> + /*
> + * We have raced with CPU hotplug. Don't do anything to avoid
> + * passing doms with offlined cpu to partition_sched_domains().
> + * Anyways, hotplug work item will rebuild sched domains.
> + */
> + if (!cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
> + return;
> +
> + /* Generate domain masks and attrs */
> + ndoms = generate_sched_domains(&doms, &attr);
> +
> + /* Have scheduler rebuild the domains */
> + partition_sched_domains(ndoms, doms, attr);
> +}
And you couldn't come up with a way to share _anything_ with the
existing rebuild_sched_domains_locked() function?
*sigh*.. please try again.
On Thu, Sep 07, 2017 at 09:28:48AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 07, 2017 at 11:34:12AM +0530, Prateek Sood wrote:
> > Remove circular dependency deadlock in a scenario where hotplug of CPU is
> > being done while there is updation in cgroup and cpuset triggered from
> > userspace.
> >
> > Example scenario:
> > kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0
> >
> > kworker/0:0 - percpu_down_write(&cpu_hotplug_lock) [held]
> > flush(work) [no high prio workqueue available on CPU]
> > wait_for_completion()
Hi Prateek,
so this is:
_cpu_down():
cpus_write_lock(); // percpu_down_write(&cpu_hotlug_lock)
cpuhp_invoke_callbacks():
workqueue_offine_cpu():
wq_update_unbound_numa():
alloc_unbound_pool():
get_unbound_pool():
create_worker():
kthread_create_on_node():
wake_up_process(kthreadd_task);
wait_for_completion(); // create->done
, right?
Wonder running in a kworker is necessary to trigger this, I mean running
a cpu_down() in a normal process context could also trigger this, no?
Just ask out of curiosity.
Regards,
Boqun
> >
> > kthreadd - percpu_down_read(cgroup_threadgroup_rwsem) [waiting]
> >
> > init:729 - percpu_down_write(cgroup_threadgroup_rwsem) [held]
> > lock(cpuset_mutex) [waiting]
> >
> > init:1 - lock(cpuset_mutex) [held]
> > percpu_down_read(&cpu_hotplug_lock) [waiting]
>
> That's both unreadable and useless :/ You want to tell what code paths
> that were, not which random tasks happened to run them.
>
>
[...]
On 09/07/2017 12:58 PM, Peter Zijlstra wrote:
> On Thu, Sep 07, 2017 at 11:34:12AM +0530, Prateek Sood wrote:
>> Remove circular dependency deadlock in a scenario where hotplug of CPU is
>> being done while there is updation in cgroup and cpuset triggered from
>> userspace.
>>
>> Example scenario:
>> kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0
>>
>> kworker/0:0 - percpu_down_write(&cpu_hotplug_lock) [held]
>> flush(work) [no high prio workqueue available on CPU]
>> wait_for_completion()
>>
>> kthreadd - percpu_down_read(cgroup_threadgroup_rwsem) [waiting]
>>
>> init:729 - percpu_down_write(cgroup_threadgroup_rwsem) [held]
>> lock(cpuset_mutex) [waiting]
>>
>> init:1 - lock(cpuset_mutex) [held]
>> percpu_down_read(&cpu_hotplug_lock) [waiting]
>
> That's both unreadable and useless :/ You want to tell what code paths
> that were, not which random tasks happened to run them.
>
>
>
>> Eliminate this dependecy by reordering locking of cpuset_mutex
>> and cpu_hotplug_lock in following order
>> 1. Acquire cpu_hotplug_lock (read)
>> 2. Acquire cpuset_mutex
>>
>> Signed-off-by: Prateek Sood <[email protected]>
>> ---
>> kernel/cgroup/cpuset.c | 70 +++++++++++++++++++++++++++++++++++++++++++-------
>> 1 file changed, 61 insertions(+), 9 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 2f4039b..687be57 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -843,10 +843,41 @@ static void rebuild_sched_domains_locked(void)
>> out:
>> put_online_cpus();
>> }
>> +
>> +/*
>> + * Rebuild scheduler domains.
>> + * Call with following lock held in the order
>> + * 1. cpu_hotplug_lock (read)
>> + * 2. cpuset_mutex
>
> Do not put that in comments, nobody ever reads comments.
>
>> + */
>> +static void rebuild_sched_domains_unlocked(void)
>
> The common postfix for a function called with the cpuhotplug lock held
> is: _cpuslocked()
>
>> +{
>> + struct sched_domain_attr *attr;
>> + cpumask_var_t *doms;
>> + int ndoms;
>
> lockdep_assert_cpus_held();
> lockdep_assert_held(&cpuset_mutex);
>
>> +
>> + /*
>> + * We have raced with CPU hotplug. Don't do anything to avoid
>> + * passing doms with offlined cpu to partition_sched_domains().
>> + * Anyways, hotplug work item will rebuild sched domains.
>> + */
>> + if (!cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>> + return;
>> +
>> + /* Generate domain masks and attrs */
>> + ndoms = generate_sched_domains(&doms, &attr);
>> +
>> + /* Have scheduler rebuild the domains */
>> + partition_sched_domains(ndoms, doms, attr);
>> +}
>
> And you couldn't come up with a way to share _anything_ with the
> existing rebuild_sched_domains_locked() function?
>
> *sigh*.. please try again.
>
Thanks for the suggestion Peter, I will resend the patch
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation
Center, Inc., is a member of Code Aurora Forum, a Linux Foundation
Collaborative Project
On 09/07/2017 02:26 PM, Boqun Feng wrote:
> On Thu, Sep 07, 2017 at 09:28:48AM +0200, Peter Zijlstra wrote:
>> On Thu, Sep 07, 2017 at 11:34:12AM +0530, Prateek Sood wrote:
>>> Remove circular dependency deadlock in a scenario where hotplug of CPU is
>>> being done while there is updation in cgroup and cpuset triggered from
>>> userspace.
>>>
>>> Example scenario:
>>> kworker/0:0 => kthreadd => init:729 => init:1 => kworker/0:0
>>>
>>> kworker/0:0 - percpu_down_write(&cpu_hotplug_lock) [held]
>>> flush(work) [no high prio workqueue available on CPU]
>>> wait_for_completion()
>
> Hi Prateek,
>
> so this is:
>
> _cpu_down():
> cpus_write_lock(); // percpu_down_write(&cpu_hotlug_lock)
> cpuhp_invoke_callbacks():
> workqueue_offine_cpu():
> wq_update_unbound_numa():
> alloc_unbound_pool():
> get_unbound_pool():
> create_worker():
> kthread_create_on_node():
> wake_up_process(kthreadd_task);
> wait_for_completion(); // create->done
>
> , right?
>
> Wonder running in a kworker is necessary to trigger this, I mean running
> a cpu_down() in a normal process context could also trigger this, no?
> Just ask out of curiosity.
>
> Regards,
> Boqun
Hi Boqun,
cpu_down() in normal process can also trigger this.
Regards
Prateek
>
>>>
>>> kthreadd - percpu_down_read(cgroup_threadgroup_rwsem) [waiting]
>>>
>>> init:729 - percpu_down_write(cgroup_threadgroup_rwsem) [held]
>>> lock(cpuset_mutex) [waiting]
>>>
>>> init:1 - lock(cpuset_mutex) [held]
>>> percpu_down_read(&cpu_hotplug_lock) [waiting]
>>
>> That's both unreadable and useless :/ You want to tell what code paths
>> that were, not which random tasks happened to run them.
>>
>>
> [...]
>
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation
Center, Inc., is a member of Code Aurora Forum, a Linux Foundation
Collaborative Project