From: "Paul E. McKenney" <[email protected]> [ Upstream commit 0bb11a372fc8d7006b4d0f42a2882939747bdbff ]
The current code will scan the entirety of each per-CPU list of exiting tasks in ->rtp_exit_list with interrupts disabled. This is normally just fine, because each CPU typically won't have very many tasks in this state. However, if a large number of tasks block late in do_exit(), these lists could be arbitrarily long. Low probability, perhaps, but it really could happen. This commit therefore occasionally re-enables interrupts while traversing these lists, inserting a dummy element to hold the current place in the list. In kernels built with CONFIG_PREEMPT_RT=y, this re-enabling happens after each list element is processed, otherwise every one-to-two jiffies. [ paulmck: Apply Frederic Weisbecker feedback. ] Link: https://lore.kernel.org/all/[email protected]/ Signed-off-by: Paul E. McKenney <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Sebastian Siewior <[email protected]> Cc: Anna-Maria Behnsen <[email protected]> Cc: Steven Rostedt <[email protected]> Signed-off-by: Boqun Feng <[email protected]> Signed-off-by: Sasha Levin <[email protected]> --- kernel/rcu/tasks.h | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index e8b72954ee106..c4ab26e77c8fa 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -968,13 +968,33 @@ static void rcu_tasks_postscan(struct list_head *hop) */ for_each_possible_cpu(cpu) { + unsigned long j = jiffies + 1; struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, cpu); struct task_struct *t; + struct task_struct *t1; + struct list_head tmp; raw_spin_lock_irq_rcu_node(rtpcp); - list_for_each_entry(t, &rtpcp->rtp_exit_list, rcu_tasks_exit_list) + list_for_each_entry_safe(t, t1, &rtpcp->rtp_exit_list, rcu_tasks_exit_list) { if (list_empty(&t->rcu_tasks_holdout_list)) rcu_tasks_pertask(t, hop); + + // RT kernels need frequent pauses, otherwise + // pause at least once per pair of jiffies. + if (!IS_ENABLED(CONFIG_PREEMPT_RT) && time_before(jiffies, j)) + continue; + + // Keep our place in the list while pausing. + // Nothing else traverses this list, so adding a + // bare list_head is OK. + list_add(&tmp, &t->rcu_tasks_exit_list); + raw_spin_unlock_irq_rcu_node(rtpcp); + cond_resched(); // For CONFIG_PREEMPT=n kernels + raw_spin_lock_irq_rcu_node(rtpcp); + t1 = list_entry(tmp.next, struct task_struct, rcu_tasks_exit_list); + list_del(&tmp); + j = jiffies + 1; + } raw_spin_unlock_irq_rcu_node(rtpcp); } -- 2.43.0
