From 23da2ad64dbe9f3fab10af90484fe41e144337b1 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Fri, 12 Jan 2024 16:46:21 +0100 Subject: rcu/exp: Remove rcu_par_gp_wq 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: __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 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 Reported-by: Thomas Gleixner Suggested-by: Joel Fernandes Suggested-by: Paul E. McKenney Suggested-by: Neeraj upadhyay Signed-off-by: Frederic Weisbecker Reviewed-by: Paul E. McKenney Signed-off-by: Boqun Feng --- kernel/rcu/tree_exp.h | 73 +-------------------------------------------------- 1 file changed, 1 insertion(+), 72 deletions(-) (limited to 'kernel/rcu/tree_exp.h') diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 0318a8a062d5..6b83537480b1 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -418,7 +418,6 @@ retry_ipi: 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 = @@ -470,69 +469,6 @@ 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. @@ -965,7 +901,6 @@ static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp) */ void synchronize_rcu_expedited(void) { - bool use_worker; unsigned long flags; struct rcu_exp_work rew; struct rcu_node *rnp; @@ -976,9 +911,6 @@ void synchronize_rcu_expedited(void) lock_is_held(&rcu_sched_lock_map), "Illegal synchronize_rcu_expedited() in RCU read-side critical section"); - use_worker = (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()) { // Note well that this code runs with !PREEMPT && !SMP. @@ -1008,7 +940,7 @@ void synchronize_rcu_expedited(void) return; /* Someone else did our work for us. */ /* Ensure that load happens before action based on it. */ - if (unlikely(!use_worker)) { + if (unlikely((rcu_scheduler_active == RCU_SCHEDULER_INIT) || !rcu_exp_worker_started())) { /* Direct call during scheduler init and early_initcalls(). */ rcu_exp_sel_wait_wake(s); } else { @@ -1025,9 +957,6 @@ void synchronize_rcu_expedited(void) /* Let the next expedited grace period start. */ mutex_unlock(&rcu_state.exp_mutex); - - if (likely(use_worker)) - synchronize_rcu_expedited_destroy_work(&rew); } EXPORT_SYMBOL_GPL(synchronize_rcu_expedited); -- cgit v1.2.3