On Mon, Oct 09, 2017 at 06:57:46PM +0530, Prateek Sood wrote:
> On 09/07/2017 11:21 PM, Peter Zijlstra wrote:
> > But if you invert these locks, the need for cpuset_hotplug_workfn() goes
> > away, at least for the CPU part, and we can make in synchronous again.
> > Yay!!
> The callback making a call to cpuset_hotplug_workfn()in hotplug path are
> [CPUHP_AP_ACTIVE] = {
> .name = "sched:active",
> .startup.single = sched_cpu_activate,
> .teardown.single = sched_cpu_deactivate,
> },
>
> if we make cpuset_hotplug_workfn() synchronous, deadlock might happen:
> _cpu_down()
> cpus_write_lock() //held
> cpuhp_kick_ap_work()
> cpuhp_kick_ap()
> __cpuhp_kick_ap()
> wake_up_process() //cpuhp_thread_fun
> wait_for_ap_thread() //wait for complete from cpuhp_thread_fun()
>
> cpuhp_thread_fun()
> cpuhp_invoke_callback()
> sched_cpu_deactivate()
> cpuset_cpu_inactive()
> cpuset_update_active_cpus()
> cpuset_hotplug_work()
> rebuild_sched_domains()
> cpus_read_lock() //waiting as acquired in _cpu_down()
Well, duh, don't use rebuild_sched_domains() 'obviously' :-) use
rebuild_sched_domains_cpuslocked() instead and it works just fine.
After applying your patch, the below boots and survives a hotplug.
---
include/linux/cpuset.h | 6 ------
kernel/cgroup/cpuset.c | 30 +++++++++---------------------
kernel/power/process.c | 2 --
kernel/sched/core.c | 1 -
4 files changed, 9 insertions(+), 30 deletions(-)
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -51,9 +51,7 @@ static inline void cpuset_dec(void)
extern int cpuset_init(void);
extern void cpuset_init_smp(void);
-extern void cpuset_force_rebuild(void);
extern void cpuset_update_active_cpus(void);
-extern void cpuset_wait_for_hotplug(void);
extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
extern void cpuset_cpus_allowed_fallback(struct task_struct *p);
extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
@@ -166,15 +164,11 @@ static inline bool cpusets_enabled(void)
static inline int cpuset_init(void) { return 0; }
static inline void cpuset_init_smp(void) {}
-static inline void cpuset_force_rebuild(void) { }
-
static inline void cpuset_update_active_cpus(void)
{
partition_sched_domains(1, NULL, NULL);
}
-static inline void cpuset_wait_for_hotplug(void) { }
-
static inline void cpuset_cpus_allowed(struct task_struct *p,
struct cpumask *mask)
{
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -833,7 +833,12 @@ static void rebuild_sched_domains_cpuslo
cpumask_var_t *doms;
int ndoms;
+ /*
+ * When called during hotplug, this lock is held by the calling
+ * thread, not cpuhp_thread_fun :/
+ *
lockdep_assert_cpus_held();
+ */
lockdep_assert_held(&cpuset_mutex);
/*
@@ -2281,13 +2286,6 @@ static void cpuset_hotplug_update_tasks(
mutex_unlock(&cpuset_mutex);
}
-static bool force_rebuild;
-
-void cpuset_force_rebuild(void)
-{
- force_rebuild = true;
-}
-
/**
* cpuset_hotplug_workfn - handle CPU/memory hotunplug for a cpuset
*
@@ -2362,25 +2360,15 @@ static void cpuset_hotplug_workfn(struct
}
/* rebuild sched domains if cpus_allowed has changed */
- if (cpus_updated || force_rebuild) {
- force_rebuild = false;
+ if (cpus_updated)
rebuild_sched_domains();
- }
}
void cpuset_update_active_cpus(void)
{
- /*
- * We're inside cpu hotplug critical region which usually nests
- * inside cgroup synchronization. Bounce actual hotplug processing
- * to a work item to avoid reverse locking order.
- */
- schedule_work(&cpuset_hotplug_work);
-}
-
-void cpuset_wait_for_hotplug(void)
-{
- flush_work(&cpuset_hotplug_work);
+ mutex_lock(&cpuset_mutex);
+ rebuild_sched_domains_cpuslocked();
+ mutex_unlock(&cpuset_mutex);
}
/*
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -203,8 +203,6 @@ void thaw_processes(void)
__usermodehelper_set_disable_depth(UMH_FREEZING);
thaw_workqueues();
- cpuset_wait_for_hotplug();
-
read_lock(&tasklist_lock);
for_each_process_thread(g, p) {
/* No other threads should have PF_SUSPEND_TASK set */
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5598,7 +5598,6 @@ static void cpuset_cpu_active(void)
* restore the original sched domains by considering the
* cpuset configurations.
*/
- cpuset_force_rebuild();
}
cpuset_update_active_cpus();
}
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