[PATCH 8/8] rcu/exp: Remove rcu_par_gp_wq

From: Frederic Weisbecker
Date: Fri Dec 08 2023 - 17:06:48 EST

TREE04 running on short iterations can produce writer stalls of the
following kind:

??? Writer stall state RTWS_EXP_SYNC(4) g3968 f0x0 ->state 0x2 cpu 0
task:rcu_torture_wri state:D stack:14568 pid:83 ppid:2 flags:0x00004000
Call Trace:
<TASK>
__schedule+0x2de/0x850
? trace_event_raw_event_rcu_exp_funnel_lock+0x6d/0xb0
schedule+0x4f/0x90
synchronize_rcu_expedited+0x430/0x670
? __pfx_autoremove_wake_function+0x10/0x10
? __pfx_synchronize_rcu_expedited+0x10/0x10
do_rtws_sync.constprop.0+0xde/0x230
rcu_torture_writer+0x4b4/0xcd0
? __pfx_rcu_torture_writer+0x10/0x10
kthread+0xc7/0xf0
? __pfx_kthread+0x10/0x10
ret_from_fork+0x2f/0x50
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1b/0x30
</TASK>

Waiting for an expedited grace period and polling for an expedited
grace period both are operations that internally rely on the same
workqueue performing necessary asynchronous work.

However, a dependency chain is involved between those two operations,
as depicted below:

====== CPU 0 ======= ====== CPU 1 =======

synchronize_rcu_expedited()
exp_funnel_lock()
mutex_lock(&rcu_state.exp_mutex);
start_poll_synchronize_rcu_expedited
queue_work(rcu_gp_wq, &rnp->exp_poll_wq);
synchronize_rcu_expedited_queue_work()
queue_work(rcu_gp_wq, &rew->rew_work);
wait_event() // A, wait for &rew->rew_work completion
mutex_unlock() // B
//======> switch to kworker

sync_rcu_do_polled_gp() {
synchronize_rcu_expedited()
exp_funnel_lock()
mutex_lock(&rcu_state.exp_mutex); // C, wait B
....
} // D

Since workqueues are usually implemented on top of several kworkers
handling the queue concurrently, the above situation wouldn't deadlock
most of the time because A then doesn't depend on D. But in case of
memory stress, a single kworker may end up handling alone all the works
in a serialized way. In that case the above layout becomes a problem
because A then waits for D, closing a circular dependency:

A -> D -> C -> B -> A

This however only happens when CONFIG_RCU_EXP_KTHREAD=n. Indeed
synchronize_rcu_expedited() is otherwise implemented on top of a kthread
worker while polling still relies on rcu_gp_wq workqueue, breaking the
above circular dependency chain.

Fix this with making expedited grace period to always rely on kthread
worker. The workqueue based implementation is essentially a duplicate
anyway now that the per-node initialization is performed by per-node
kthread workers.

Meanwhile the CONFIG_RCU_EXP_KTHREAD switch is still kept around to
manage the scheduler policy of these kthread workers.

Reported-by: Anna-Maria Behnsen <anna-maria.behnsen@xxxxxxxxxxxxx>
Reported-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Suggested-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Suggested-by: Neeraj upadhyay <Neeraj.Upadhyay@xxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
kernel/rcu/rcu.h | 4 ---
kernel/rcu/tree.c | 40 ++++---------------------
kernel/rcu/tree.h | 4 ---
kernel/rcu/tree_exp.h | 70 +------------------------------------------
4 files changed, 7 insertions(+), 111 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 6beaf70d629f..99032b9cb667 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -623,11 +623,7 @@ int rcu_get_gp_kthreads_prio(void);
void rcu_fwd_progress_check(unsigned long j);
void rcu_force_quiescent_state(void);
extern struct workqueue_struct *rcu_gp_wq;
-#ifdef CONFIG_RCU_EXP_KTHREAD
extern struct kthread_worker *rcu_exp_gp_kworker;
-#else /* !CONFIG_RCU_EXP_KTHREAD */
-extern struct workqueue_struct *rcu_par_gp_wq;
-#endif /* CONFIG_RCU_EXP_KTHREAD */
void rcu_gp_slow_register(atomic_t *rgssp);
void rcu_gp_slow_unregister(atomic_t *rgssp);
#endif /* #else #ifdef CONFIG_TINY_RCU */
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e75ddf42e9b1..0c28adb56ad4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4367,7 +4367,6 @@ rcu_boot_init_percpu_data(int cpu)
rcu_boot_init_nocb_percpu_data(rdp);
}

-#ifdef CONFIG_RCU_EXP_KTHREAD
struct kthread_worker *rcu_exp_gp_kworker;

static void rcu_spawn_exp_par_gp_kworker(struct rcu_node *rnp)
@@ -4387,7 +4386,9 @@ static void rcu_spawn_exp_par_gp_kworker(struct rcu_node *rnp)
return;
}
WRITE_ONCE(rnp->exp_kworker, kworker);
- sched_setscheduler_nocheck(kworker->task, SCHED_FIFO, &param);
+
+ if (IS_ENABLED(CONFIG_RCU_EXP_KTHREAD))
+ sched_setscheduler_nocheck(kworker->task, SCHED_FIFO, &param);
}

static struct task_struct *rcu_exp_par_gp_task(struct rcu_node *rnp)
@@ -4411,39 +4412,14 @@ static void __init rcu_start_exp_gp_kworker(void)
rcu_exp_gp_kworker = NULL;
return;
}
- sched_setscheduler_nocheck(rcu_exp_gp_kworker->task, SCHED_FIFO, &param);
-}
-
-static inline void rcu_alloc_par_gp_wq(void)
-{
-}
-#else /* !CONFIG_RCU_EXP_KTHREAD */
-struct workqueue_struct *rcu_par_gp_wq;
-
-static void rcu_spawn_exp_par_gp_kworker(struct rcu_node *rnp)
-{
-}
-
-static struct task_struct *rcu_exp_par_gp_task(struct rcu_node *rnp)
-{
- return NULL;
-}
-
-static void __init rcu_start_exp_gp_kworker(void)
-{
-}

-static inline void rcu_alloc_par_gp_wq(void)
-{
- rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
- WARN_ON(!rcu_par_gp_wq);
+ if (IS_ENABLED(CONFIG_RCU_EXP_KTHREAD))
+ sched_setscheduler_nocheck(rcu_exp_gp_kworker->task, SCHED_FIFO, &param);
}
-#endif /* CONFIG_RCU_EXP_KTHREAD */

static void rcu_spawn_rnp_kthreads(struct rcu_node *rnp)
{
- if ((IS_ENABLED(CONFIG_RCU_EXP_KTHREAD) ||
- IS_ENABLED(CONFIG_RCU_BOOST)) && rcu_scheduler_fully_active) {
+ if (rcu_scheduler_fully_active) {
mutex_lock(&rnp->kthread_mutex);
rcu_spawn_one_boost_kthread(rnp);
rcu_spawn_exp_par_gp_kworker(rnp);
@@ -4527,9 +4503,6 @@ static void rcutree_affinity_setting(unsigned int cpu, int outgoingcpu)
struct rcu_node *rnp;
struct task_struct *task_boost, *task_exp;

- if (!IS_ENABLED(CONFIG_RCU_EXP_KTHREAD) && !IS_ENABLED(CONFIG_RCU_BOOST))
- return;
-
rdp = per_cpu_ptr(&rcu_data, cpu);
rnp = rdp->mynode;

@@ -5209,7 +5182,6 @@ void __init rcu_init(void)
/* Create workqueue for Tree SRCU and for expedited GPs. */
rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
WARN_ON(!rcu_gp_wq);
- rcu_alloc_par_gp_wq();

/* Fill in default value for rcutree.qovld boot parameter. */
/* -After- the rcu_node ->lock fields are initialized! */
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index ef3d3385063f..35f7af331e6c 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -24,11 +24,7 @@
/* Communicate arguments to a workqueue handler. */
struct rcu_exp_work {
unsigned long rew_s;
-#ifdef CONFIG_RCU_EXP_KTHREAD
struct kthread_work rew_work;
-#else
- struct work_struct rew_work;
-#endif /* CONFIG_RCU_EXP_KTHREAD */
};

/* RCU's kthread states for tracing. */
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 744d6acf5553..dd33948ab80f 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -420,7 +420,6 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)

static void rcu_exp_sel_wait_wake(unsigned long s);

-#ifdef CONFIG_RCU_EXP_KTHREAD
static void sync_rcu_exp_select_node_cpus(struct kthread_work *wp)
{
struct rcu_exp_work *rewp =
@@ -472,69 +471,6 @@ static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew
kthread_queue_work(rcu_exp_gp_kworker, &rew->rew_work);
}

-static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
-{
-}
-#else /* !CONFIG_RCU_EXP_KTHREAD */
-static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
-{
- struct rcu_exp_work *rewp =
- container_of(wp, struct rcu_exp_work, rew_work);
-
- __sync_rcu_exp_select_node_cpus(rewp);
-}
-
-static inline bool rcu_exp_worker_started(void)
-{
- return !!READ_ONCE(rcu_gp_wq);
-}
-
-static inline bool rcu_exp_par_worker_started(struct rcu_node *rnp)
-{
- return !!READ_ONCE(rcu_par_gp_wq);
-}
-
-static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
-{
- int cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);
-
- INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
- /* If all offline, queue the work on an unbound CPU. */
- if (unlikely(cpu > rnp->grphi - rnp->grplo))
- cpu = WORK_CPU_UNBOUND;
- else
- cpu += rnp->grplo;
- queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
-}
-
-static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
-{
- flush_work(&rnp->rew.rew_work);
-}
-
-/*
- * Work-queue handler to drive an expedited grace period forward.
- */
-static void wait_rcu_exp_gp(struct work_struct *wp)
-{
- struct rcu_exp_work *rewp;
-
- rewp = container_of(wp, struct rcu_exp_work, rew_work);
- rcu_exp_sel_wait_wake(rewp->rew_s);
-}
-
-static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
-{
- INIT_WORK_ONSTACK(&rew->rew_work, wait_rcu_exp_gp);
- queue_work(rcu_gp_wq, &rew->rew_work);
-}
-
-static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
-{
- destroy_work_on_stack(&rew->rew_work);
-}
-#endif /* CONFIG_RCU_EXP_KTHREAD */
-
/*
* Select the nodes that the upcoming expedited grace period needs
* to wait for.
@@ -978,8 +914,7 @@ void synchronize_rcu_expedited(void)
lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_rcu_expedited() in RCU read-side critical section");

- can_queue = (rcu_scheduler_active != RCU_SCHEDULER_INIT) &&
- rcu_exp_worker_started();
+ can_queue = (rcu_scheduler_active != RCU_SCHEDULER_INIT) && rcu_exp_worker_started();

/* Is the state is such that the call is a grace period? */
if (rcu_blocking_is_gp()) {
@@ -1027,9 +962,6 @@ void synchronize_rcu_expedited(void)

/* Let the next expedited grace period start. */
mutex_unlock(&rcu_state.exp_mutex);
-
- if (likely(can_queue))
- synchronize_rcu_expedited_destroy_work(&rew);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

--
2.42.1